scripts/bootstrap-prefix: make darwin bootstrap with clang

[plxty]

In Darwin there's plenty of tools that now use macOS infra such as nodejs, many rust libraries... For GCC lacks of macOS builtins and the blocks extension, building many tools require a lot of hack, switching to clang will reduce some pain.

This is still a proof-of-concept style, it works on my macOS 26, but may not be a clean way, and may not work for other macOS version.

Changes explaination:

1. Darwin ships with builtin libc++, for Gentoo clang with isolated libc++ to work, changes required: compiler flag -stdlib++-isystem to use Gentoo headers, and building libcxx requires -DLIBCXX_ABI_NAMESPACE= to avoid conflicting the system one.
2. So in stage 2 we still use macOS libcxx, then in stage3 we shall emerge the libcxx and switching toolchain like ninja and rest to use the Gentoo lib.
3. Uniform STAGE and BOOTSTRAP_STAGE, but I'm not sure if that's the correct way :/
4. Adding bash requirement check for array stuff like ( xxx ) to work

Still this is a proof-of-concept, managed to get bootstrap but may encounter other issues.

[thesamesam]

It looks good to me.