[AMDGPU][clang] Definition of gfx11 subtarget

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

Patch 2/N for upstreaming of AMDGPU gfx11 architecture

Depends on D124536

Reviewed By: foad, kzhuravl, #amdgpu, arsenm

Differential Revision: https://reviews.llvm.org/D124537