[FPEnv] [Clang] Enable constrained FP support for PowerPC

d4ce862f introduced HasStrictFP to disable generating constrained FP
operations for platforms lacking support. Since work for enabling
constrained FP on PowerPC is almost done, we'd like to enable it.

Reviewed By: kpn, steven.zhang

Differential Revision: https://reviews.llvm.org/D87223