libstdc++: Check autoconf macros for strtof and strtold [PR110653]

libstdc++: Check autoconf macros for strtof and strtold [PR110653]

As well as the _GLIBCXX_USE_C99_STDLIB check, we also have a separate
check in linkage.m4 for just strtof and strtold. We can use that to
declare std::strtof and std::strtold in <cstdlib> for additional
targets. That allows us to enable std::stold on hpux11.11 which is
missing strtoll, strtoull and strtof, so doesn't define
_GLIBCXX_USE_C99_STDLIB. Although it doesn't help hpux11.11, we can
define std::stof for more targets this way too.

As with the previous commit for PR110653, this only affects the narrow
character overloads. std::stof and std::stold for wstring still requires
C99 <wchar.h> support.

libstdc++-v3/ChangeLog:

	PR libstdc++/110653
	* include/bits/basic_string.h [_GLIBCXX_HAVE_STRTOF] (stof):
	Define.
	[_GLIBCXX_HAVE_STRTOLD] (stold): Define.
	* include/c_global/cstdlib [_GLIBCXX_HAVE_STRTOF] (strtof):
	Declare in namespace std.
	[_GLIBCXX_HAVE_STRTOLD] (strtold): Likewise.