[AMDGPU] Update libc/runtime target to use amdgpu (#210032)
Update libc/runtime targets to use "amdgpu" instead of "amdgcn". This
change is necessary to do libc testing with a
check-libc-amdgpu-amd-amdhsa target. The previous
check-libc-amdgcn-amd-amdhsa target already stopped working after
https://github.com/llvm/llvm-project/pull/209247.
Signed-off-by: John Lu <John.Lu@amd.com>