lang/luajit requires a much more modern compiler than the gcc4.2.1 available
on non-clang architechtures. In addition, if there is no lang/gcc available
for the architecuture, we can't build it via that method either.

Rework archtecture disable to use ONLY_FOR_ARCHS and give a clearer reason
why this is being disabled on powrepc/mips/sparc64.

PR: 197132
Reviewed by: osa@freebsd.org (maintainer)