[testsuite] tolerate missing std::stold

[testsuite] tolerate missing std::stold

basic_string.h doesn't define the non-w string version of std::stold
when certain conditions aren't met, and then a couple of tests fail to
compile.

Guard the portions of the tests that depend on std::stold with the
conditions for it to be defined.

for  libstdc++-v3/ChangeLog

	* testsuite/21_strings/basic_string/numeric_conversions/char/stold.cc:
	Guard non-wide stold calls with conditions for it to be
	defined.
	* testsuite/27_io/basic_ostream/inserters_arithmetic/char/hexfloat.cc:
	Likewise.