runtime: remove overlap in the GC CPU pause time metrics

Currently the GC CPU pause time metrics start measuring before the STW
is complete. This results in a slightly less accurate measurement and
creates some overlap with other timings (for example, the idle time of
idle Ps) that will cause double-counting.

This CL adds a field to worldStop to track the point at which the world
actually stopped and uses that as the basis for the GC CPU pause time
metrics, basically eliminating this overlap.

Note that this will cause Ps in _Pgcstop before the world is fully
stopped to be counted as user time. A follow-up CL will fix this
discrepancy.

Change-Id: I287731f08415ffd97d327f582ddf7e5d2248a6f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/570258
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>

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