AMDGPU: [AMDGPU] Assembler: add .hsa_code_object_metadata directive for functime metadata V2.0

Summary:
Added pair of directives .hsa_code_object_metadata/.end_hsa_code_object_metadata.
Between them user can put YAML string that would be directly put to the generated note. E.g.:
'''
.hsa_code_object_metadata
    {
        amd.MDVersion: [ 2, 0 ]
    }
.end_hsa_code_object_metadata
'''
Based on D25046

Reviewers: vpykhtin, nhaustov, yaxunl, tstellarAMD

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, mgorny, tony-tye

Differential Revision: https://reviews.llvm.org/D27619

llvm-svn: 290097