1) Move non-preemption check even earlier in newstack.
This avoids a few priority inversion problems.

2) Always use atomic operations to update bitmap for 1-word objects.
This avoids lost mark bits during concurrent GC.

3) Stop using work.nproc == 1 as a signal for being single-threaded.
The concurrent GC runs with work.nproc == 1 but other procs are
running mutator code.

The use of work.nproc == 1 in getfull *is* safe, but remove it anyway,
since it is saving only a single atomic operation per GC round.

Fixes #9225.

Change-Id: I24134f100ad592ea8cb59efb6a54f5a1311093dc
Reviewed-on: https://go-review.googlesource.com/2745Reviewed-by: Rick Hudson <rlh@golang.org>