AMDGPU: Emit runtime metadata as a note element in .note section

Currently runtime metadata is emitted as an ELF section with name .AMDGPU.runtime_metadata.

However there is a standard way to convey vendor specific information about how to run an ELF binary, which is called vendor-specific note element (http://www.netbsd.org/docs/kernel/elf-notes.html).

This patch lets AMDGPU backend emits runtime metadata as a note element in .note section.

Differential Revision: https://reviews.llvm.org/D25781

llvm-svn: 286502