[AMDGPU] Update code object metadata format documentation

* Add amdhsa prefix to names to allow other tools to use the metadata
  without collision.
* Make names consistent.
* Simplify structure.
* Change note record ID.
* Switch from YAML to MsgPack format.
* Document metadata assembler directive.

Patch By: t-tye (Tony Tye)
Differential Revision: https://reviews.llvm.org/D53445



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346992 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed