devel/kBuild: fix build on powerpc64

powerpc64 needs new GCC to build this port (not base GCC). KBUILD_ARCH needs to be corrected, in the current form it ends up as ppc3264, which is not we want.

PR: 239161
Approved by: linimon (mentor), vbox (maintainer timeout)
Differential Revision: https://reviews.freebsd.org/D21083