runtime: flush mcaches to mcentral before reading memstats

Currently mcaches are flushed to mcentral after a bunch of memstats have
already been read. This is not safe (in the sense that it doesn't ensure
consisent memstats) since memstats may in general change when mcentral
data structures are manipulated.

Note that prior to the new mcentral implementation this was not a
problem because mcentral operations happened to never modify certain
memstats. As of the new mcentral implementation, we might for example
persistentalloc when uncaching a span, which would change memstats. This
can cause a skew between the value of sys (which currently is calculated
before mcaches are flushed) and the value of gc_sys and other_sys.

Fix this by moving mcache flushing to the very top of updatememstats.
Also leave a comment explaining that this must be done first, in
general, because mcentrals make no guarantee that they will not
influence memstats (and doing so would be unnecessarily restrictive).

Fixes #38712.

Change-Id: I15bacb313c54a46e380a945a71bb75db67169c1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/230498


Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>