[AMDGPU] Added MsgPack format PAL metadata

Summary:
PAL metadata now supports both the old linear reg=val pairs format and
the new MsgPack format.

The MsgPack format uses YAML as its textual representation. On output to
YAML, a mnemonic name is provided for some hardware registers.

Differential Revision: https://reviews.llvm.org/D57028

Change-Id: I2bbaabaaca4b3574f7e03b80fbef7c7a69d06a94
llvm-svn: 356591