runtime: report GC CPU utilization in MemStats

This adds a GCCPUFraction field to MemStats that reports the
cumulative fraction of the program's execution time spent in the
garbage collector. This is equivalent to the utilization percent shown
in the gctrace output and makes this available programmatically.

This does make one small effect on the gctrace output: we now report
the duration of mark termination up to just before the final
start-the-world, rather than up to just after. However, unlike
stop-the-world, I don't believe there's any way that start-the-world
can block, so it should take negligible time.

While there are many statistics one might want to expose via MemStats,
this is one of the few that will undoubtedly remain meaningful
regardless of future changes to the memory system.

The diff for this change is larger than the actual change. Mostly it
lifts the code for computing the GC CPU utilization out of the
debug.gctrace path.

Updates #10323.

Change-Id: I0f7dc3fdcafe95e8d1233ceb79de606b48acd989
Reviewed-on: https://go-review.googlesource.com/12844


Reviewed-by: Russ Cox <rsc@golang.org>