Currently windows crashes because early allocs in schedinit
try to allocate tiny memory blocks, but m->p is not yet setup.
I've considered calling procresize(1) earlier in schedinit,
but this refactoring is better and must fix the issue as well.
Fixes #7218.

R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/54570045