libstdc++: Fix up abi.exp FAILs on powerpc64le-linux

This is an ABI problem on powerpc64le-linux, introduced in 13.1.
When libstdc++ is configured against old glibc, the
_ZSt10from_charsPKcS0_RDF128_St12chars_format@@GLIBCXX_3.4.31
_ZSt8to_charsPcS_DF128_@@GLIBCXX_3.4.31
_ZSt8to_charsPcS_DF128_St12chars_format@@GLIBCXX_3.4.31
_ZSt8to_charsPcS_DF128_St12chars_formati@@GLIBCXX_3.4.31
symbols are exported from the library, while when it is configured against
new enough glibc, those symbols aren't exported and we export instead
_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format@@GLIBCXX_IEEE128_3.4.29
_ZSt8to_charsPcS_u9__ieee128@@GLIBCXX_IEEE128_3.4.29
_ZSt8to_charsPcS_u9__ieee128St12chars_format@@GLIBCXX_IEEE128_3.4.29
_ZSt8to_charsPcS_u9__ieee128St12chars_formati@@GLIBCXX_IEEE128_3.4.29
together with various other @@GLIBCXX_IEEE128_3.4.{29,30,31} and
@@CXXABI_IEEE128_1.3.13 symbols.  The idea was that those *IEEE128* symbol
versions (similarly to *LDBL* symbol versions) are optional (but if it
appears, all symbols from it up to the version of the library appears),
but the base appears always.
My _Float128 from_chars/to_chars changes unfortunately broke this.
I believe nothing really uses those symbols if libstdc++ has been
configured against old glibc, so if 13.1 wasn't already released, it might
be best to make sure they aren't exported on powerpc64le-linux.
But as they were exported, I think the best resolution for this ABI
difference is to add those 4 symbols as aliases to the
GLIBCXX_IEEE128_3.4.29 *u9__ieee128* symbols, which the following patch
does.

2023-05-03  Jakub Jelinek  <jakub@redhat.com>

	* src/c++17/floating_from_chars.cc
	(_ZSt10from_charsPKcS0_RDF128_St12chars_format): New alias to
	_ZSt10from_charsPKcS0_Ru9__ieee128St12chars_format.
	* src/c++17/floating_to_chars.cc (_ZSt8to_charsPcS_DF128_): New alias to
	_ZSt8to_charsPcS_u9__ieee128.
	(_ZSt8to_charsPcS_DF128_St12chars_format): New alias to
	_ZSt8to_charsPcS_u9__ieee128St12chars_format.
	(_ZSt8to_charsPcS_DF128_St12chars_formati): New alias to
	_ZSt8to_charsPcS_u9__ieee128St12chars_formati.
	* config/abi/post/powerpc64le-linux-gnu/baseline_symbols.txt: Updated.

(cherry picked from commit b51e2fd6)