libstdc++: Fix simd compilation with Clang

Clang fails to compile some constant expressions involving simd.
Therefore, just disable this non-conforming extension for clang.

Fix AVX512 blend implementation for Clang. It was converting the bitmask
to bool before, which is obviously wrong. Instead use a Clang builtin to
convert the bitmask to vector-mask before using a vector blend ?:. A
similar change is required for the masked unary implementation, because
the GCC builtins do not exist on Clang.

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

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd_detail.h: Don't declare the
	simd API as constexpr with Clang.
	* include/experimental/bits/simd_x86.h (__movm): New.
	(_S_blend_avx512): Resolve FIXME. Implement blend using __movm
	and ?:.
	(_SimdImplX86::_S_masked_unary): Clang does not implement the
	same builtins. Implement the function using __movm, ?:, and -
	operators on vector_size types instead.

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