[PowerPC] Add option for ROP Protection
Added -mrop-protection for Power PC to turn on codegen that provides some
protection from ROP attacks.
The option is off by default and can be turned on for Power 8, Power 9 and
This patch is for the option only. The feature will be implemented by a later
Reviewed By: amyk
Differential Revision: https://reviews.llvm.org/D96512