native-xtools: Ensure GCC files are cleaned up.

Because we force enable MK_GCC when building we need to also force
enable it for the cleaning phase. Otherwise the NXB_TARGET files
are found in the next build's kernel-toolchain phase and cause
an error.

Reported by: sbruno
X-MFC-With: r325001
MFC after: 1 month
Sponsored by: Dell EMC