This patch adds the support to restrict prefixed instruction from
crossing the 64 byte boundary:
- Add the infrastructure to register a custom XCOFF streamer
- Add a custom XCOFF streamer for PowerPC to allow us to
intercept instructions as they are being emitted and align all 8 byte
instructions to a 64 byte boundary if required by adding a 4 byte nop.
Reviewed By: stefanp
Differential Revision: https://reviews.llvm.org/D101107
Victor Huang