runtime: select GC mark workers during start-the-world

When the GC starts today, procresize and startTheWorldWithSema don't
consider the additional Ps required to run the mark workers. procresize
and startTheWorldWithSema resume only the Ps necessary to run the normal
user goroutines.

Once those Ps start, findRunnable and findRunnableGCWorker determine
that a GC worker is necessary and run the worker instead, calling wakep
to wake another P to run the original user goroutine.

This is unfortunate because it disrupts the intentional placement of Ps
on Ms that procresize does. It also has the unfortunate side effect of
slightly delaying start-the-world time, as it takes several sequential
wakeps to get all Ps started.

To address this, procresize explicitly assigns GC mark workers to Ps
before starting the world. The assignment occurs _after_ selecting
runnable Ps, so that we prefer to select Ps that were previously idle.

Note that if fewer than 25% of Ps are idle then we won't be able to
assign all dedicated workers, and some of the Ps intended for user
goroutines will convert to dedicated workers once they reach
findRunnableGCWorker.

Also note that stack scanning temporarily suspends the goroutine. Resume
occurs through ready, which will move the goroutine to the local runq of
the P that did the scan. Thus there is still a source of migration at
some point during the GC.

For #65694.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest
Change-Id: I6a6a636c51f39f4f4bc716aa87de68f6ebe163a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/721002
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

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