libstdc++: Remove handling for underscore-prefixed libm functions [PR111638]

libstdc++: Remove handling for underscore-prefixed libm functions [PR111638]

The checks in linkage.m4 try to support math functions prefixed with
underscores, like _acosf and _isinf. However, that doesn't work because
they're renamed to the standard names using a macro, but then <cmath>
undefines that macro again.

This simply removes everything related to those underscored functions.

libstdc++-v3/ChangeLog:

	PR libstdc++/111638
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* linkage.m4 (GLIBCXX_MAYBE_UNDERSCORED_FUNCS): Remove.
	(GLIBCXX_CHECK_MATH_DECL_AND_LINKAGE_1): Do not check for _foo.
	(GLIBCXX_CHECK_MATH_DECLS_AND_LINKAGES_1): Likewise.
	(GLIBCXX_CHECK_MATH_DECL_AND_LINKAGE_2): Likewise.
	(GLIBCXX_CHECK_MATH_DECL_AND_LINKAGE_3): Likewise.
	(GLIBCXX_CHECK_STDLIB_DECL_AND_LINKAGE_2): Do not use
	GLIBCXX_MAYBE_UNDERSCORED_FUNCS.