libstdc++: Skip integer division optimization for Clang

libstdc++: Skip integer division optimization for Clang

Clang ICEs on _SimdImplX86::_S_divides. The function is only working
around a missed optimization and not necessary for correctness.
Therefore, don't use it for Clang.

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

	* include/experimental/bits/simd_detail.h: Don't define
	_GLIBCXX_SIMD_WORKAROUND_PR90993 for Clang.
	* include/experimental/bits/simd_x86.h (_S_divides): Remove
	check for __clang__.