Revert "[PowerPC] [Clang] Enable float128 feature on VSX targets"

This commit brought build break in some f128 related tests. But that's
not the root cause. There exists some differences between Clang and
GCC's definition for 128-bit float types on PPC, so macros/functions in
glibc may not work with clang -mfloat128 well. We need to handle this
carefully and reland it.