buildworld already runs the stand build in parallel, so make it easier to identify ordering issues by properly establishing dependencies or adding .WAIT where needed.
Everything in stand/ relies on libsa, either directly or indirectly, because libsa build is where the stand headers get installed and it gets linked in most places.
Interpreters depend on their libs, machine dirs usually depend on top-level libs that are getting built and at least one of the interpreter flavors.
For i386, order btx/libi386/libfirewire before everything else using a big-ol-.WAIT hammer. btx is the most common dependency, but the others are used sporadically. This seems to be where the race reporting on the mailing list is- AFAICT, the following sequence is happening:
1.) One of the loaders gets built based on stale btx/btxldr
2.) btx/btxldr gets rebuilt
3.) installworld triggers loader rebuild because btx was rebuilt after
This seems like the most plausible explanation, as they've verified system time and timestamps.
While we're here, let's switch stand/ over to a completely parallel build so we can work out these kinds of issues in isolation rather than in the middle of a larger build.
 I'm still working out how it accomplishes this, but reports on the mailing list seem to indicate that some sprinkled SUBDIR_DEPEND fixes the ordering issues; stand/Makefile doesn't seem to be invoked with SUBDIR_PARALLEL defined, but I suspect there's magic afoot arising from the theory that buildworld strictly orders things such that all dependencies should have been built in earlier stages and it may parallelize the rest.