[PowerPC] Fix FLT_ROUNDS_ on little endian

In lowering of FLT_ROUNDS_, FPSCR content will be moved into FP register
and then GPR, and then truncated into word.

For subtargets without direct move support, it will store and then load.
The load address needs adjustment (+4) only on big-endian targets. This
patch fixes it on using generic opcodes on little-endian and subtargets
with direct-move.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D91845