Clean up pthread startup code (NFC)

[kripken]

There were two places that pthread workers were initialized, one in a postSet,
and one at the end of postamble. This joins the two into a single initWorker
method, and moves code to there, which is simpler.

(Also add a missing initWorker for minimal runtime, so this is technically
not NFC for that runtime. Note that there may be more fixes needed there,
this just does the obvious thing for this PR to improve things.)

This will make fixing a race condition in a subsequent PR easier.

[sbc100]

What was wrong with doing the init work in postset?

[kripken]

Nothing wrong with a postset, this is just a cleanup. One place doing initialization is cleaner than two I think.

[sbc100]

Isn't it cleaner to do it in postset and avoid any code in postable/preamble completely?

(Full disclaimer I don't really get how postset works).

[kripken]

Oh, if we unify them then it can't be in a postset. A postset is emitted together with the JS library deps of a thing. That is, it ends up in some "random" location in the middle (as we emit the JS library things while we parse what is needed and recursively parse their deps and so forth - deterministic, but if you add another import from the JS library the entire order can change). The other mechanism has a clear fixed position, which is near the end of the entire file, and after necessary things (like JS library stuff) all exist. So if we unify them, it has to be in the other place, and not the postset.