crossdirect: calling "cc" from foreign arch chroot fails

When trying to compile !1472 (merged) for aarch64, it fails with:

ccache: error: Could not find compiler "aarch64-alpine-linux-musl-cc" in PATH

The problem is, that ccache and crossdirect provies a aarch64-alpine-linux-musl-cc binary, but GCC does not.

First I thought that the binary used to exist in GCC before the upgrade to 10 that happened recently in Alpine. But that's not the case (3.12 is on GCC-9):

• https://pkgs.alpinelinux.org/contents?file=x86_64*&path=&name=gcc&branch=edge
• https://pkgs.alpinelinux.org/contents?file=x86_64*&path=&name=gcc&branch=v3.12

My initial fix attempt of removing "cc" from crossdirect and ccache-cross-symlinks pmaports (!1520 (closed)) would just result in not using the cross compilers if "cc" was specified as compiler.

I'll work on a proper fix.