[libcxx] [libcxxabi] [libunwind] [libcxx][CI] Prepare ARM Picolib builds for upgrade to clang 21 (PR #158300)

[Victor Campos via cfe-commits]

vhscampos wrote:

LGTM. Just a few typos: it's "picolibc" rather than "picolib". Missing the final 'c'.

https://github.com/llvm/llvm-project/pull/158300