[PowerPC] Unaligned FP default should apply to scalars only

As reported in PR45186, we could be in a situation where we don't
want to handle unaligned memory accesses for FP scalars but still
have VSX (which allows unaligned access for vectors). Change the
default to only apply to scalars.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45186
(cherry picked from commit 099a875f)