runtime: fix gctrace=1
runtime: fix gctrace=1

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://codereview.appspot.com/116880043

Committer: Dmitriy Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

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