This adds support for the new __ieee128 long double format on
powerpc64le targets.

Most of the complexity comes from wanting a single libstdc++.so library
that contains the symbols needed by code compiled with both
-mabi=ibmlongdouble and -mabi=ieeelongdouble (and not forgetting
-mlong-double-64 as well!)

In a few places this just requires an extra overload, for example
std::from_chars has to be overloaded for both forms of long double.
That can be done in a single translation unit that defines overloads
for 'long double' and also '__ieee128', so that user code including
<charconv> will be able to link to a definition for either type of long
double. Those are the easy cases.

The difficult parts are (as for the std::string ABI transition) the I/O
and locale facets. In order to be able to write either form of long
double to an ostream such as std::cout we need the locale to contain a
std::num_put facet that can handle both forms. The same approach is
taken as was already done for supporting 64-bit long double and 128-bit
long double: adding extra overloads of do_put to the facet class. On
targets where the new long double code is enabled, the facets that are
registered in the locale at program startup have additional overloads so
that they can work with any long double type. Where this fails to work
is if user code installs its own facet, which will probably not have the
additional overloads and so will only be able to output one or the other
type. In practice the number of users expecting to be able to use their
own locale facets in code using a mix of -mabi=ibmlongdouble and
-mabi=ieeelongdouble is probably close to zero.

libstdc++-v3/ChangeLog:

	* Makefile.in: Regenerate.
	* config.h.in: Regenerate.
	* config/abi/pre/gnu.ver: Make patterns less greedy.
	* config/os/gnu-linux/ldbl-ieee128-extra.ver: New file with patterns
	for IEEE128 long double symbols.
	* configure: Regenerate.
	* configure.ac: Enable alternative 128-bit long double format on
	powerpc64*-*-linux*.
	* doc/Makefile.in: Regenerate.
	* fragment.am: Regenerate.
	* include/Makefile.am: Set _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT.
	* include/Makefile.in: Regenerate.
	* include/bits/c++config: Define inline namespace for new long
	double symbols. Don't define _GLIBCXX_USE_FLOAT128 when it's the
	same type as long double.
	* include/bits/locale_classes.h [_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT]
	(locale::_Impl::_M_init_extra_ldbl128): Declare new member function.
	* include/bits/locale_facets.h (_GLIBCXX_NUM_FACETS): Simplify by
	only counting narrow character facets.
	(_GLIBCXX_NUM_CXX11_FACETS): Likewise.
	(_GLIBCXX_NUM_LBDL_ALT128_FACETS): New.
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT] (num_get::__do_get): Define
	vtable placeholder for __ibm128 long double type.
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(num_get::__do_get): Declare vtable placeholder for __ibm128 long
	double type.
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(num_put::__do_put): Likewise.
	* include/bits/locale_facets.tcc
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(num_get::__do_get, num_put::__do_put): Define.
	* include/bits/locale_facets_nonio.h
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(money_get::__do_get): Declare vtable placeholder for __ibm128 long
	double type.
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(money_put::__do_put): Likewise.
	* include/bits/locale_facets_nonio.tcc
	[_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT && __LONG_DOUBLE_IEEE128__]
	(money_get::__do_get, money_put::__do_put): Define.
	* include/ext/numeric_traits.h [_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT]
	(__numeric_traits<__ibm128>, __numeric_traits<__ieee128>): Define.
	* libsupc++/Makefile.in: Regenerate.
	* po/Makefile.in: Regenerate.
	* python/Makefile.in: Regenerate.
	* src/Makefile.am: Add compatibility-ldbl-alt128.cc and
	compatibility-ldbl-alt128-cxx11.cc sources and recipes for objects.
	* src/Makefile.in: Regenerate.
	* src/c++11/Makefile.in: Regenerate.
	* src/c++11/compatibility-ldbl-alt128-cxx11.cc: New file defining
	symbols using the old 128-bit long double format, for the cxx11 ABI.
	* src/c++11/compatibility-ldbl-alt128.cc: Likewise, for the
	gcc4-compatible ABI.
	* src/c++11/compatibility-ldbl-facets-aliases.h: New header for long
	double compat aliases.
	* src/c++11/cow-locale_init.cc: Add comment.
	* src/c++11/cxx11-locale-inst.cc: Define C and C_is_char
	unconditionally.
	* src/c++11/cxx11-wlocale-inst.cc: Add sanity check. Include
	locale-inst.cc directly, not via cxx11-locale-inst.cc.
	* src/c++11/locale-inst-monetary.h: New header for monetary
	category instantiations.
	* src/c++11/locale-inst-numeric.h: New header for numeric category
	instantiations.
	* src/c++11/locale-inst.cc: Include new headers for monetary,
	numeric, and long double definitions.
	* src/c++11/wlocale-inst.cc: Remove long double compat aliases that
	are defined in new header now.
	* src/c++17/Makefile.am: Use -mabi=ibmlongdouble for
	floating_from_chars.cc.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++17/floating_from_chars.cc (from_chars_impl): Add
	if-constexpr branch for __ieee128.
	(from_chars): Overload for __ieee128.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++98/Makefile.in: Regenerate.
	* src/c++98/locale_init.cc (num_facets): Adjust calculation.
	(locale::_Impl::_Impl(size_t)): Call _M_init_extra_ldbl128.
	* src/c++98/localename.cc (num_facets): Adjust calculation.
	(locale::_Impl::_Impl(const char*, size_t)): Call
	_M_init_extra_ldbl128.
	* src/filesystem/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/util/testsuite_abi.cc: Add new symbol versions.
	Allow new symbols to be added to GLIBCXX_IEEE128_3.4.29 and
	CXXABI_IEEE128_1.3.13 too.
	* testsuite/26_numerics/complex/abi_tag.cc: Add u9__ieee128 to
	regex matching expected symbols.