runtime: clean up async preemption loose ends

[aclements]

Go 1.14 introduces asynchronous preemption so that tight loops can be preempted (#10958). However, there are still several loose ends to tie up. This is a general bug to keep track of remaining work:

• Support on all possible platforms (js/wasm is not currently possible)
  • windows/arm (CL 207961)
  • windows/arm64
  • darwin/arm (no longer supported)
  • plan9/* (CL 228893)
• Redo unsafe-point representation so we can eliminate register maps (CL 230544)
• Remove register maps and extra stack maps (after redesigning debug call injection) (CL 230544)
• Possibly incorporate @cherrymui's sequence restarting (CL 208126)
• Attach unsafety/restartability to Progs and generate the stream post-assembly like spadj (comment)
• Make large pointer-free memmoves/memclrs preemptible (runtime: long pauses STW (sweep termination) on massive block allocation #31222)
• Make large pointer-full memmoves/memclrs preemptible (this is a little harder)
• Make large allocations preemptible (closely related to the above two, partly done)
• Fix various annoying spins in the scheduler
• Make more of the runtime preemptible

[gopherbot]

Change https://golang.org/cl/229298 mentions this issue: runtime: make newproc1 not start the goroutine

[gopherbot]

Change https://golang.org/cl/229300 mentions this issue: runtime: use conservative scanning for debug calls

[gopherbot]

Change https://golang.org/cl/229299 mentions this issue: runtime: perform debug call injection on a new goroutine

[gopherbot]

Change https://golang.org/cl/230539 mentions this issue: cmd/compile: mark PanicBounds/Extend as calls

[gopherbot]

Change https://golang.org/cl/230542 mentions this issue: cmd/compile: don't emit stack maps for write barrier calls

[gopherbot]

Change https://golang.org/cl/230544 mentions this issue: cmd/compile,runtime: stack maps only at calls, remove register maps

[gopherbot]

Change https://golang.org/cl/230541 mentions this issue: cmd/compile: fix unsafe-points with stack maps

[gopherbot]

Change https://golang.org/cl/230540 mentions this issue: cmd/compile: rename issafepoint -> hasStackMap

[gopherbot]

Change https://golang.org/cl/230543 mentions this issue: cmd/compile: make LivenessMap sparse