libstdc++: ppc: conditionalize vsx-only simd intrinsics

libstdc++: ppc: conditionalize vsx-only simd intrinsics

libstdc++'s bits/simd.h section for PowerPC, guarded by __ALTIVEC__,
defines various intrinsic vector types that are only available with
__VSX__: 64-bit long double, double, (un)signed long long, and 64-bit
(un)signed long.

experimental/simd/standard_abi_usable{,_2}.cc tests error out
reporting the unmet requirements when the target cpu doesn't enable
VSX.  Make the reported instrinsic types conditional on __VSX__ so
that <experimental/simd> can be used on PowerPC variants that do not
support VSX.

for  libstdc++-v3/ChangeLog

	* include/experimental/bits/simd.h [__ALTIVEC__]: Require VSX
	for double, long long, and 64-bit long intrinsic types.
	[__ALTIVEC__] (__intrinsic_type): Mention 128-bit in
	preexisting long double diagnostic, adjust no-VSX double
	diagnostic to cover 64-bit long double as well.