libstdc++: Resolve -Wunused-variable warnings in stdx::simd and tests

libstdc++: Resolve -Wunused-variable warnings in stdx::simd and tests

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd_builtin.h (_S_fpclassify): Move
	__infn into #ifdef'ed block.
	* testsuite/experimental/simd/tests/fpclassify.cc: Declare
	constants only when used.
	* testsuite/experimental/simd/tests/frexp.cc: Likewise.
	* testsuite/experimental/simd/tests/logarithm.cc: Likewise.
	* testsuite/experimental/simd/tests/trunc_ceil_floor.cc:
	Likewise.
	* testsuite/experimental/simd/tests/ldexp_scalbn_scalbln_modf.cc:
	Move totest and expect1 into #ifdef'ed block.