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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Will get someone from Arm to review as well since they are consumers of Picolib and libcxx.

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