These memstats are currently being computed by gcMark, which was
appropriate in Go 1.4, but gcMark is now just one part of a bigger
picture. In particular, it can't account for the sweep termination
pause time, it can't account for all of the mark termination pause
time, and the reported "pause end" and "last GC" times will be
slightly earlier than they really are.

Lift computing of these statistics into func gc, which has the
appropriate visibility into the process to compute them correctly.

Fixes one of the issues in #10323. This does not add new statistics
appropriate to the concurrent collector; it simply fixes existing
statistics that are being misreported.

Change-Id: I670cb16594a8641f6b27acf4472db15b6e8e086e
Reviewed-on: https://go-review.googlesource.com/11794Reviewed-by: Russ Cox <rsc@golang.org>