gcc: only build go for the final toolchain
gcc: only build go for the final toolchain

The subpackages etc. are not defined when not CHROOT_READY, and
it is not strictly necessary to have gccgo in the initial gcc,
as it can be compiled without having an existing gccgo toolchain.

[ci skip]

necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages