runtime: abstract initializing and destroying Ps out of procresize
procresize is rather ungainly because it includes all of the code to
initialize new Ps and tear down unused Ps. Pull these out into their
own methods on the p type. This also tweaks the initialization loop in
procresize so we only initialize new Ps (and Ps we previously
destroyed) rather than asking each P "have you been initialized?"
This is for #10958 and #24543, but it's also just a nice cleanup.
Change-Id: Ic1242066f572c94a23cea8ea4dc47c918e31d559
Reviewed-on: https://go-review.googlesource.com/c/go/+/171762 Reviewed-by: Michael Knyszek <mknyszek@google.com>