libstdc++: Fix simd test compilation with Clang

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	* testsuite/experimental/simd/tests/operators.cc: Clang doesn't
	define __GCC_IEC_559. Use __STDC_IEC_559__ instead.