[PowerPC] Allow -mfloat128 option for VSX targets

Targets with VSX feature but without native float128 instructions can
also use that type with supplementary libcalls. We don't enable it by
default now because Glibc assumes long double and float128 can be
implicitly converted in between, which is not available under default
'ibmlongdouble' semantics in clang.

This commit partly relands cbd93cee.