updatememstats is called on both the m and g stacks.
Call into flushallmcaches correctly.  flushallmcaches
can only run on the M stack.

This is somewhat temporary.  once ReadMemStats is in
Go we can have all of this code M-only.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/116880043