[llvm] [NVPTX] Use .common linkage for common globals (PR #84416)

[Artem Belevich via llvm-commits]

Artem-B wrote:

@MaskRay can you double check that PTX's idea of common matches what LLVM promises for `GVar->hasCommonLinkage()`? 

The patch LGTM otherwise.

https://github.com/llvm/llvm-project/pull/84416