runtime: add option to scavenge with lock held throughout

runtime: add option to scavenge with lock held throughout

This change adds a "locked" parameter to scavenge() and scavengeone()
which allows these methods to be run with the heap lock acquired, and
synchronously with respect to others which acquire the heap lock.

This mode is necessary for both heap-growth scavenging (multiple
asynchronous scavengers here could be problematic) and
debug.FreeOSMemory.

Updates #35112.

Change-Id: I24eea8e40f971760999c980981893676b4c9b666
Reviewed-on: https://go-review.googlesource.com/c/go/+/195699
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>