runtime: reduce GC assist extra credit

Mutator goroutines that allocate memory during the concurrent mark
phase are required to spend some time assisting the garbage
collector. The magnitude of this mandatory assistance is proportional
to the goroutine's allocation debt and subject to the assistance
ratio as calculated by the pacer.

When assisting the garbage collector, a mutator goroutine will go
beyond paying off its allocation debt. It will build up extra credit
to amortize the overhead of the assist.

In fast-allocating applications with high assist ratios, building up
this credit can take the affected goroutine's entire time slice.
Reduce the penalty on each goroutine being selected to assist the GC
in two ways, to spread the responsibility more evenly.

First, do a consistent amount of extra scan work without regard for
the pacer's assistance ratio. Second, reduce the magnitude of the
extra scan work so it can be completed within a few hundred
microseconds.

Commentary on gcOverAssistWork is by Austin Clements, originally in
https://golang.org/cl/24704

Updates #14812
Fixes #16432

Change-Id: I436f899e778c20daa314f3e9f0e2a1bbd53b43e1
Reviewed-on: https://go-review.googlesource.com/25155


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>