[llvm] [offload] [test] Skip amdgcn/nvptx tests if detected arch is not built (PR #119006)

[Shilei Tian via llvm-commits]

https://github.com/shiltian commented:

A potentially better alternative would be to define a CMake variable when a target is enabled, and then check if the variable exists and is true when enabling tests.

https://github.com/llvm/llvm-project/pull/119006