runtime: refactor hmap setoverflow into newoverflow

This simplifies the code, as well as providing
a single place to modify to change the
allocation of new overflow buckets.

Updates #19931
Updates #19992

Change-Id: I77070619f5c8fe449bbc35278278bca5eda780f2
Reviewed-on: https://go-review.googlesource.com/40975


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>