libstdc++: Fix mask reduction of simd_mask<double> on POWER7

libstdc++: Fix mask reduction of simd_mask<double> on POWER7

POWER7 does not support __vector long long reductions, making the
generic _S_popcount implementation ill-formed. Specializing _S_popcount
for PPC allows optimization and avoids the issue.

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h: Add __have_power10vec
	conditional on _ARCH_PWR10.
	* include/experimental/bits/simd_builtin.h: Forward declare
	_MaskImplPpc and use it as _MaskImpl when __ALTIVEC__ is
	defined.
	(_MaskImplBuiltin::_S_some_of): Call _S_popcount from the
	_SuperImpl for optimizations and correctness.
	* include/experimental/bits/simd_ppc.h: Add _MaskImplPpc.
	(_MaskImplPpc::_S_popcount): Implement via vec_cntm for POWER10.
	Otherwise, for >=int use -vec_sums divided by a sizeof factor.
	For <int use -vec_sums(vec_sum4s(...)) to sum all mask entries.