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

[Michał Górny via llvm-commits]

mgorny wrote:

Does this version look better? Or should I move the condition straight into `DeviceRTL` and just pass two booleans from there?

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