libstdc++: testsuite: require cmath for exp simd

libstdc++: testsuite: require cmath for exp simd

simd_math.h assumes declarations for many C99 functions to be present,
that libstdc++ doesn't add to target systems that don't have them in
the C library.

Add the C99 math requirement to tests for simd features, so that they
don't fail because of limitations of the target C library.

for  libstdc++-v3/ChangeLog

	* testsuite/experimental/simd/standard_abi_usable.cc: Require
	cmath support.
	* testsuite/experimental/simd/standard_abi_usable_2.cc:
	Likewise.