[clang] [AMDGPU] Mark int64_t and intmax_t as LP64 (PR #214865)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> This looks right for standalone AMDGCN, but I’m worried about the aux-target path.

This code path isn't reached for the aux targets, it's only the direct AMDGPU target.

https://github.com/llvm/llvm-project/pull/214865