[AMDGPU] llvm-objdump: Skip amd_kernel_code_t only at the begining of kernel symbol.

Summary: This change fix bug in AMDGPU disassembly. Previously, presence of symbols other than kernel symbols caused objdump to skip begining of those symbols.

Reviewers: tstellarAMD, vpykhtin, Bigcheese, ruiu

Subscribers: kzhuravl, arsenm

Differential Revision: http://reviews.llvm.org/D21966

llvm-svn: 278921