[AMDGPU] add labels to +DumpCode output

Summary:
+DumpCode is a hack to embed disassembly in the ELF file. This commit
fixes it to include labels, to make it slightly more useful.

Reviewers: arsenm, kzhuravl

Subscribers: nhaehnle, timcorringham, dstuttard, llvm-commits, t-tye, yaxunl, wdng, kzhuravl

Differential Revision: https://reviews.llvm.org/D40169

llvm-svn: 320146