native-xtools: Fix build without META_MODE for GCC archs.

The initial kernel-toolchain is built with TARGET=MACHINE but
we want GCC to have files generated for TARGET=NXB_TARGET
instead later on. Just clean the files between building of
the toolchain and nxb binaries which will let it rebuild
when needed.

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