[AMDGPU] Update code object description

[AMDGPU] Update code object description

 - Use ELF header flags to identify processor.
 - Remove isa note record.
 - Add target feature section.
 - Make metadata for NumVGPRs, NumSGPRs and MaxFlatWorkGroupSize required.
 - Add FixedWorkGroupSize to CodeProps metadata.
 - Add ReqdWorkGroupSize* to kernel descriptor and move MaxFlatWorkGroupSize to be adjacent.
 - Move IsXNACKEnabled in the kernel descriptor to be at the end of the unused flags.
 - Remove IsDynamicCallStack from the metadata and kernel descriptor.
 - Remove legacy debugger metadata.
 - Remove old XNACK enabled processor names.

Differential Revision: https://reviews.llvm.org/D39828

llvm-svn: 317855