build: parallelize Makefile jobs by default

[agilgur5]

Motivation

• optimize some of the make jobs by parallelizing them by default when possible
  • this way you don't have to specify, for instance, make start -j4 UI=true etc

Modifications

• -j flags allows it to start more processes

  • e.g. -j4 means it can start 4

• use nprocs to get number of available CPUs

Verification

Used the Makefile with this unstaged change for a few PRs, confirmed some things ran in parallel

• the logs are interleaved for parallel jobs by default, so fairly simple to tell

[agilgur5]

Well that's interesting, CI codegen failed. I did actually run make codegen with this change (for #12582), but probably wasn't fresh like it is on GH Actions. Might be an ordering issue / missing dependent job in the Makefile then

[agilgur5]

Ok everything else passed, so it's just codegen failing. I think that might be because of the rm -rf v3 lines -- there are several individual jobs that have that and if they ran while another codegen job was running, that would conflict.

Either need to remove those (replace with only one at the end of codegen for instance) or make codegen sequential (not as ideal).

Kicking this back into draft for now as I need to test that a bit and am working on some higher priority things (P1 fixes)