[PowerPC] Change default for unaligned FP access for older subtargets

This is a fix for https://bugs.llvm.org/show_bug.cgi?id=40554

Some CPU's trap to the kernel on unaligned floating point access and there are
kernels that do not handle the interrupt. The program then fails with a SIGBUS
according to the PR. This just switches the default for unaligned access to only
allow it on recent server CPUs that are known to allow this.

Differential revision: https://reviews.llvm.org/D71954