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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Arm colleagues alerted me to the fact that picolibc does have some code to handle this `ALIGN_WITH_INPUT` thing when lld is used. Either CI is using a too old verison of picolibc, or that code is not triggering.
> 
> To keep each change simple, I'm going ahead with this PR, then I'll work out if we can in fact get lld working and send another PR if so.

I added LLD support to picolibc in https://github.com/picolibc/picolibc/pull/376, so it should all work out of the box.

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