runtime/pprof: fix block profile bias

Block profiles were biased towards infrequent long events over frequent
short events. This fix corrects the bias by aggregating shorter events
as longer but less frequent in the profiles. As a result their
cumulative duration will be accurately represented in the profile
without skewing their sample mean (duration/count).

Credit to @dvyukov for suggesting to adjust the count in the
saveblockevent function.

Fixes #44192.

Change-Id: I71a99d7f6ebdb2d484d44890a2517863cceb4004
Reviewed-on: https://go-review.googlesource.com/c/go/+/299991


Trust: Michael Pratt <mpratt@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>