emulators/qemu{,70}: enable for aarch64

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=ppc64, 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.

Approved by: bofh (maintainer)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37234