runtime: add pollDesc partial edges

gscan is taken during stack growth, which may occur while pollDesc is
held.

mallocgc may also be called while pollDesc is held. mallocgc may take
mheap or mheapSpecial. The former exists, but is out of order; the
latter is missing.

Fixes #44881

Change-Id: Ie25935d9d433e813c11a528ee47255b317a09f41
Reviewed-on: https://go-review.googlesource.com/c/go/+/300009


Trust: Michael Pratt <mpratt@google.com>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>