Piggy-back off of the work done for powerpc64 to enable aarch64. ppc64
was untested, though I did sanity check MAKE_ARGS with ARCH forced to
both powerpc64 and powerpc64le. Both result in ARCH=ppc, as they did
before.
While we're here, enable ${MAKE_JOBS_NUMBER} support -- the meson build
will carry any -j flag passed to gmake on to ninja, otherwise it will
default to -j1.
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