xz-bootstrap uses clang for PPC on Rosetta

[barracuda156]

This problem only concerns xz-bootstrap, not regular xz (the latter correctly uses gcc-4.2).

The port still [pretends to] build – with multiple warnings:

clang: warning: not using the clang compiler for the 'powerpc' architecture

This is a wrong behavior, clang should never be invoked for ppc builds.

[ryandesign (Ryan Carsten Schmidt)]

This applies to all the -bootstrap ports.

It happens because the -bootstrap ports specify this:

    compiler.whitelist      clang llvm-gcc-4.2 gcc-4.2 apple-gcc-4.2

Josh added that in [e967bb1325decd8d0c05f2f37b27067f6d780eb3/macports-ports] to fix #58876.