mp_startup_common races itself
Review Request #1173 — Created Aug. 21, 2018 and submitted
Fixes mp startup so that we don't startup a core until we've finished starting up the previous core.
We've tested this on the hardware used at Joyent, as well as my Broadwell-based personal server (which was really good at triggering the problem). Additionally, people have tested the images with the fix and not reported any boot problems due to it.