Simplify some of the logic for external GCC toolchain packages.

• Always define BU_PREFIX as GCC_TARGET if needed. This was already effectively true, but BU_PREFIX was just being set to the same value in the case that GCC_TARGET wasn't defined. This is simpler and avoids duplicating some logic.
• Simplify amd64-gcc by not defining GCC_TARGET and BU_PREFIX in that port but instead replacing "amd64" with "x86_64" in the devel/powerpc64-gcc Makefile. This matches what base/gcc does.
• Remove FREEBSD_EXTENSION now that it is no longer needed.

Approved by: zeising
Differential Revision: https://reviews.freebsd.org/D18349