Fix pmbootstrap aportgen gcc-*

    After recent refactoring of the upstream gcc package, running tracedeps
    on our generated gcc cross packages did not work anymore:
    
      >>> ERROR: gcc-aarch64*: libc.musl-aarch64.so.1: path not found
    
    The gcc packages were manually adjusted (thanks Luca and Clayton!) to
    not use tracedeps, and the dependencies were added manually as
    workaround. Do the same programmatically in pmbootstrap aportgen code
    for now, so generating new cross aports works again.
    
    This is actually better than using tracedeps, because with the new
    method the gcc cross packages don't immediately break anymore when gcc
    gets changed in Alpine.

See individual commits for details.

Close #2517 (closed)