[AMDGPU] Add gfx11 subtarget ELF definition

This is the first patch of a series to upstream support for the new
subtarget.

Contributors:
Jay Foad <jay.foad@amd.com>
Konstantin Zhuravlyov <kzhuravl_dev@outlook.com>

Patch 1/N for upstreaming AMDGPU gfx11 architectures.

Reviewed By: foad, kzhuravl, #amdgpu

Differential Revision: https://reviews.llvm.org/D124536