[libclc] [NFC][libclc] Set MACRO_ARCH to ${ARCH} uncondionally before customizing (PR #156789)

[Fraser Cormack via cfe-commits]

https://github.com/frasercrmck approved this pull request.

LGTM. I don't like `set` repetition in `if`/`else` either. Not least because it's worse for downstreams.

https://github.com/llvm/llvm-project/pull/156789