[FPEnv] PowerPC-specific builtin constrained FP enablement

This change enables PowerPC compiler builtins to generate constrained
floating point operations when clang is indicated to do so.

A couple of possibly unexpected backend divergences between constrained
floating point and regular behavior are highlighted under the test tag
FIXME-CHECK. This may be something for those on the PPC backend to look
at.

Patch by: Drew Wock <drew.wock@sas.com>

Differential Revision: https://reviews.llvm.org/D82020