If gcAssistAlloc is unable to steal or perform enough scan work, it
calls timeSleep, which allocates. If this allocation requires
obtaining a new span, it will in turn attempt to assist GC. Since
there's likely still no way to satisfy the assist, it will sleep
again, and so on, leading to potentially deep (not infinite, but also
not bounded) recursion.

Fix this by disallowing assists during the timeSleep.

This same problem was fixed on master by 65aa2da. That commit built on
several other changes and hence can't be directly cherry-picked. This
commit implements the same idea.

Fixes #12894.

Change-Id: I152977eb1d0a3005c42ff3985d58778f054a86d4
Reviewed-on: https://go-review.googlesource.com/15720
Reviewed-by: Rick Hudson <rlh@golang.org>

Commit c257dfb attempted to fix recursive allocation in gcAssistAlloc;
however, it only reduced it: setting gp.gcalloc to 0 isn't sufficient
to disable assists at the beginning of the GC cycle when gp.gcscanwork
is also small or zero.

Fix this recursion more completely by setting gcalloc to a sentinel
value that directly disables assists.

Fixes #12894 (again).

Change-Id: I9599566222d8f540d0b39806846bfc702e6666e5
Reviewed-on: https://go-review.googlesource.com/15891
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>