[AMDGPU] Set metadata access for explicit section

Summary:
This patch provides a means to set Metadata section kind
for a global variable, if its explicit section name is
prefixed with ".AMDGPU.metadata."
This could be useful to make the global variable go to
an ELF section without any section flags set.

Reviewers: dstuttard, tpr, kzhuravl, nhaehnle, t-tye

Reviewed By: dstuttard, kzhuravl

Subscribers: llvm-commits, arsenm, jvesely, wdng, yaxunl, t-tye

Differential Revision: https://reviews.llvm.org/D55267

llvm-svn: 348922