Set .ORDER for makesyscalls generated files

[kevans]

Summary

When either makesyscalls.lua or syscalls.master changes, all of the ${GENERATED} targets are now out-of-date.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 D23098, 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.

[kevans]

After discussing with sjg, I'm going to update these to .ORDER: ${GENERATED} and add inline comments describing why we care.

[kevans]

Update to use .ORDER, hopefully articulate the problem a little better both in commit message and actually add a comment lightly mentioning the problem we're addressing.

[imp]

Looks good, with the possible exception of one new file added to GENERATED...

sys/kern/Makefile

16

Is this intentional?

[kevans]

sys/kern/Makefile

16

Whoops, I forgot to rebase -- I actually already committed that part last night since we should run the script again if somehow systrace_args.c is out of date but for some reason the others aren't.

[brooks]

Looks fine, I agree with the comment on irc/slack that centralizing this in a .mk file likely makes sense. It's no much code, but it's a lot of duplication.