There are at least 3 bugs:
1. g->stacksize accounting is broken during copystack/shrinkstack
2. stktop->free is not properly maintained during copystack/shrinkstack
3. stktop->free logic is broken:
        we can have stktop->free==FixedStack,
        and we will free it into stack cache,
        but it actually comes from heap as the result of non-copying segment shrink
This shows as at least spurious races on race builders (maybe something else as well I don't know).

The idea behind the refactoring is to consolidate stacksize and
segment origin logic in stackalloc/stackfree.

Fixes #7490.

LGTM=rsc, khr
R=golang-codereviews, rsc, khr
CC=golang-codereviews
https://golang.org/cl/72440043