[llvm] [AMDGPU][NFC] Update tests to use -triple= instead of -arch=. (PR #84153)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks! Only a few tests woul fail if -march is changed to -mtriple, presumably due to some target triple caused data layout issues.
```
% rg 'march=(r600|amdgpu)' -l
CodeGen/AMDGPU/fneg-fabs.ll
CodeGen/AMDGPU/schedule-if-2.ll
CodeGen/AMDGPU/floor.ll
CodeGen/AMDGPU/fabs.ll
CodeGen/AMDGPU/r600-infinite-loop-bug-while-reorganizing-vector.ll
CodeGen/AMDGPU/complex-folding.ll
```

https://github.com/llvm/llvm-project/pull/84153