Pull up following revision(s) (requested by riastradh in ticket #655):

sys/modules/Makefile: revision 1.285
share/mk/bsd.own.mk: revision 1.1365
share/mk/bsd.own.mk: revision 1.1366
sys/arch/aarch64/include/sljit_machdep.h: revision 1.4
sys/external/bsd/sljit/dist/sljit_src/sljitNativeARM_64.c: revision 1.5

sljit: Pacify -Wsign-compare.

If these sizes are negative, we're probably in trouble anyway, so
assert nonnegative here.
Needed to resolve PR 58103.

bsd.own.mk: Enable MKLSJIT on aarch64.

Make sure there's only one copy of the conditional, in bsd.own.mk;
just make sys/modules/Makefile conditional on MKSLJIT so we don't
have to keep these in sync.

As a workaround for PR 58106, tweak the conditional definition of
SLJIT_CACHE_FLUSH to use cpu_icache_sync_range only in _HARDKERNEL,
and use __builtin___clear_cache in userland and in rump kernels.

PR 58103: bpfjit.kmod is not built on aarch64
bsd.own.mk: No need for MKSLJIT to be set differently from others.
- Use ?=, not =, so mk.conf setting wins.
- Write out per-architecture tabular settings, not a conditional.
- Add comments for the architectures that look like they should have
  sljit but don't.  (XXX Missing comments about powerpc and mips --
  not sure why, is this because modules don't yet work on those
  architectures, or what?)

Tidying for PR 58103: bpfjit.kmod is not built on aarch64.

(martin)