[AMDGPU] Factored PAL metadata handling out into its own class

Summary:
This commit introduces a new AMDGPUPALMetadata class that:
* is inside the AMDGPU target;
* keeps an in-memory representation of PAL metadata;
* provides a method to read the frontend-supplied metadata from LLVM IR;
* provides methods for the asm printer to set metadata items;
* provides methods to write the metadata as a binary blob to put in a
  .note record or as an asm directive;
* provides a method to read the metadata as a binary blob from a .note
  record.

Because llvm-readobj cannot call directly into a target, I had to remove
llvm-readobj's ability to dump PAL metadata, pending a resolution to
https://reviews.llvm.org/D52821

Differential Revision: https://reviews.llvm.org/D57027

Change-Id: I756dc830894fcb6850324cdcfa87c0120eb2cf64
llvm-svn: 356582