[llvm] [AMDGPU,test] Change llc -march= to -mtriple= (PR #75982)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> I think we should just make the behavior of -march not stupidly depend on the host, and default to arch-unknown-unknown

This is what `-mtriple=xxx` does. `-march=` just switches the architecture but keeps the OS/environment. This is by design.

https://github.com/llvm/llvm-project/pull/75982