[libcxx][ci] In picolib build, ask clang for the normalised triple (#90722)

This is needed for a workaround to make sure the link later succeeds. I
don't know the reason for that but it is definitely needed.

https://github.com/llvm/llvm-project/pull/89234 will/wants to correct
the triple normalisation for -none- and this means that clang prior to
19, and clang 19 and above will have different answers and therefore
different library paths.

I don't want to bootstrap a clang just for libcxx CI, or require that
anyone building for Arm do the same, so ask the compiler what the triple
should be.

This will be compatible with 17 and 19 when we do update to that
version.

I'm assuming $CC is what anyone locally would set to override the
compiler, and `cc` is the binary name in our CI containers. It's not
perfect but it should cover most use cases.

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