libstdc++: More efficient masked inc-/decrement implementation

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

libstdc++-v3/ChangeLog:

	PR libstdc++/108856
	* include/experimental/bits/simd_builtin.h
	(_SimdImplBuiltin::_S_masked_unary): More efficient
	implementation of masked inc-/decrement for integers and floats
	without AVX2.
	* include/experimental/bits/simd_x86.h
	(_SimdImplX86::_S_masked_unary): New. Use AVX512 masked subtract
	builtins for masked inc-/decrement.