libstdc++: Fix test on x86_64 and non-simd targets

libstdc++: Fix test on x86_64 and non-simd targets

* Running a test compiled with AVX512 instructions requires
avx512f_runtime not just avx512f.

* The 'reduce2' test violated an invariant of fixed_size_simd_mask and
thus failed on all targets without 16-Byte vector builtins enabled (in
bits/simd.h).

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

	PR libstdc++/115575
	* testsuite/experimental/simd/pr115454_find_last_set.cc: Require
	avx512f_runtime. Don't memcpy fixed_size masks.