[clang] [spirv][amdgpu] Set atomic size in the clang target info (PR #128569)

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx approved this pull request.

LGTM, thanks. FWIW we've only done HIP bring-up, so there will probably be a few of these lingering.

https://github.com/llvm/llvm-project/pull/128569