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

[Michał Górny via llvm-commits]

mgorny wrote:

Hmm, except that `plugins-nextgen` is built before `DeviceRTL`, so we don't have the list there yet. No clue if they can be safely reordered.

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