runtime: make sure heapBitsBulkBarrier cannot be preempted
Changes the torture test in #12068 from failing about 1/10 times
to not failing in almost 2,000 runs.
This was only happening in -race mode because functions are
bigger in -race mode, so a few of the helpers for heapBitsBulkBarrier
were not being inlined, and they were not marked nosplit,
so (only in -race mode) the write barrier was being preempted by GC,
causing missed pointer updates.
Filed issue #12069 for diagnosis of any other similar errors.