[AMDGPU] implemented pal metadata

Summary:
For the amdpal OS type:

We write an AMDGPU_PAL_METADATA record in the .note section in the ELF
(or as an assembler directive). It contains key=value pairs of 32 bit
ints. It is a merge of metadata from codegen of the shaders, and
metadata provided by the frontend as _amdgpu_pal_metadata IR metadata.
Where both sources have a key=value with the same key, the two values
are ORed together.

This .note record is part of the amdpal ABI and will be documented in
docs/AMDGPUUsage.rst in a future commit.

Eventually the amdpal OS type will stop generating the .AMDGPU.config
section once the frontend has safely moved over to using the .note
records above instead of .AMDGPU.config.

Reviewers: arsenm, nhaehnle, dstuttard

Subscribers: kzhuravl, wdng, yaxunl, llvm-commits, t-tye

Differential Revision: https://reviews.llvm.org/D37753

llvm-svn: 314829