Currently, there are three ways to satisfy a GC assist: 1) the mutator
steals credit from background GC, 2) the mutator actually does GC
work, and 3) there is no more work available. 3 was never really
intended as a way to satisfy an assist, and it causes problems: there
are periods when it's expected that the GC won't have any work, such
as when transitioning from mark 1 to mark 2 and from mark 2 to mark
termination. During these periods, there's no back-pressure on rapidly
allocating mutators, which lets them race ahead of the heap goal.

For example, test/init1.go and the runtime/trace test both have small
reachable heaps and contain loops that rapidly allocate large garbage
byte slices. This bug lets these tests exceed the heap goal by several
orders of magnitude.

Fix this by forcing the assist (and hence the allocation) to block
until it can satisfy its debt via either 1 or 2, or the GC cycle
terminates.

This fixes one the causes of #11677. It's still possible to overshoot
the GC heap goal, but with this change the overshoot is almost exactly
by the amount of allocation that happens during the concurrent scan
phase, between when the heap passes the GC trigger and when the GC
enables assists.

Change-Id: I5ef4edcb0d2e13a1e432e66e8245f2bd9f8995be
Reviewed-on: https://go-review.googlesource.com/12671Reviewed-by: Russ Cox <rsc@golang.org>