runtime: move mem profile sampling into m-acquired section

It was not safe to do mcache profiling updates outside the critical
section, but we got lucky because the runtime was not preemptible.
Adding chunked memory clearing (CL 270943) created preemption
opportunities, which led to corruption of runtime data structures.

Fixes #47304.
Fixes #47302.

Change-Id: I461615470d62328a83ccbac537fbdc6dcde81c85
Reviewed-on: https://go-review.googlesource.com/c/go/+/336449


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>