c++: Change the mangling of __alignof__ [PR88115]

This patch changes the mangling of __alignof__ to v111__alignof__,
making its mangling distinct from that of alignof(type) and
alignof(expr).

How we mangle ALIGNOF_EXPR now depends on its ALIGNOF_EXPR_STD_P flag,
which after the previous patch gets consistently set for alignof(type)
as well as alignof(expr).

gcc/c-family/ChangeLog:

	PR c++/88115
	* c-opts.c (c_common_post_options): Update latest_abi_version.

gcc/ChangeLog:

	PR c++/88115
	* common.opt (-fabi-version): Document =15.
	* doc/invoke.texi (C++ Dialect Options): Likewise.

gcc/cp/ChangeLog:

	PR c++/88115
	* mangle.c (write_expression): Mangle __alignof_ differently
	from alignof when the ABI version is at least 15.

libiberty/ChangeLog:

	PR c++/88115
	* cp-demangle.c (d_print_comp_inner)
	<case DEMANGLE_COMPONENT_EXTENDED_OPERATOR>: Don't print the
	"operator " prefix for __alignof__.
	<case DEMANGLE_COMPONENT_UNARY>: Always print parens around the
	operand of __alignof__.
	* testsuite/demangle-expected: Test demangling for __alignof__.

gcc/testsuite/ChangeLog:

	PR c++/88115
	* g++.dg/abi/macro0.C: Adjust.
	* g++.dg/cpp0x/alignof7.C: New test.
	* g++.dg/cpp0x/alignof8.C: New test.