MFC r355473, r356540, r356604, r356868, r356937: sysent improvements

The main motivation here being .ORDER to render -jN > 1 harmless; svr4/ibcs2
targets were also refactored a bit, but both are irregular and cannot use
sysent.mk as-is due to differences in files generated. I have no interest in
refactoring, since these are gone in head anyways.

r355473: sysent: Reduce duplication and improve readability.

Use the power of variable to avoid spelling out source and generated
files too many times. The previous Makefiles were hard to read, hard to
edit, and badly formatted.

r356540:
kern/Makefile: systrace_args.c is also generated

r356604:
Set .ORDER for makesyscalls generated files

When either makesyscalls.lua or syscalls.master changes, all of the
${GENERATED} targets are now out-of-date. With make jobs > 1, this means we
will run the makesyscalls script in parallel for the same ABI, generating
the same set of output files.

Prior to r356603 , there is a large window for interlacing output for some
of the generated files that we were generating in-place rather than staging
in a temp dir. After that, we still should't need to run the script more
than once per-ABI as the first invocation should update all of them. Add
.ORDER to do so cleanly.

r356868:
sysent targets: further cleanup and deduplication

r355473 vastly improved the readability and cleanliness of these Makefiles.
Every single one of them follows the same pattern and duplicates the exact
same logic.

Now that we have GENERATED/SRCS, split SRCS up into the two parameters we'll
use for ${MAKESYSCALLS} rather than assuming a specific ordering of SRCS and
include a common sysent.mk to handle the rest. This makes it less tedious to
make sweeping changes.

Some default values are provided for GENERATED/SYSENT_*; almost all of these
just use a 'syscalls.master' and 'syscalls.conf' in cwd, and they all use
effectively the same filenames with an arbitrary prefix. Most ABIs will be
able to get away with just setting GENERATED_PREFIX and including
^/sys/conf/sysent.mk, while others only need light additions. kern/Makefile
is the notable exception, as it doesn't take a SYSENT_CONF and the generated
files are spread out between ^/sys/kern and ^/sys/sys, but it otherwise fits
the pattern enough to use the common version.

r356937:
sysent.mk: split interpreter out of target command

The main objective here is to make it easy to identify what needs to change
in order to use a different sysent generator than the current Lua-based one,
which may be used to MFC some of the changes that have happened so we can
avoid parallel accidents in stable branches, for instance.

As a secondary objective, it's now feasible to override the generator on a
per-Makefile basis if needed, so that one could refactor their Makefile to
use this while pinning generation to the legacy makesyscalls.sh. I don't
anticipate any consistent need for such a thing, but it's low-effort to
achieve.