[Offload] Correctly reject building on unsupported architectures (#92276)

Summary:
Previously we had this `LIBOMPTARGET_ENABLED` variable which controlled
including `libomptarget`. This is now redundant since it's controlled by
`LLVM_ENABLE_RUNTIMES`. However, this had the extra effect of not
building it when given unsupported targets. THis was lost during the
move to `offload`. This patch moves this logic back and makes the
`offload` target just quit without doing anything if used on an
unsupported architecture.

https://github.com/llvm/llvm-project/issues/91881
https://github.com/llvm/llvm-project/issues/91819



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Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>