[bug#54239] [PATCH] gnu: Add cross-clang.

[Julien Lepiller]

Hi Guix!

This small patch series adds cross-clang, a cross-compiler version of
clang. Clang doesn't really make a distinction between a native and a
cross-build, it is already a cross-compiler, but this ensures that:

1. it actually works
2. it targets (%current-target-architecture) by default

The rest of the series ensures that libcxx and libcxxabi can be
cross-compiled with it.