libstdc++: Check _GLIBCXX_USE_C99_STDLIB for strtof and strtold

libstdc++: Check _GLIBCXX_USE_C99_STDLIB for strtof and strtold

On broken systems we only have strtod, not strtof and strtold. Just use
strtod for all types, even though that will produce incorrect results in
some cases.

Similarly, if _GLIBCXX_USE_C99_MATH is not defined then std::isinf won't
be declared. Just refer to it unqualified, which should find the C
library's isinf macro if that hasn't been #undef'd by <cmath>.

libstdc++-v3/ChangeLog:

	* src/c++17/floating_from_chars.cc (from_chars_impl): Use
	isinf unqualified.
	[!_GLIBCXX_USE_C99_STDLIB]: Use strtod for float and long
	double.