Re-do the FreeBSD cross-builds to use Clang and libc++. Fixes #44433.

The main goal here is to use FreeBSD's normal libc++, instead of
statically linking the libstdc++ packaged with GCC, because that
libstdc++ has bugs that cause rustc to deadlock inside LLVM.

But the easiest way to use libc++ is to switch the build from GCC to
Clang, and the Clang package in the Ubuntu image already knows how to
cross-compile (given a sysroot and preferably cross-binutils), so the
toolchain script now uses that instead of building a custom compiler.

This also de-duplicates the `build-toolchain.sh` script.

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