runtime: use signals to preempt Gs for suspendG

This adds support for pausing a running G by sending a signal to its
M.

The main complication is that we want to target a G, but can only send
a signal to an M. Hence, the protocol we use is to simply mark the G
for preemption (which we already do) and send the M a "wake up and
look around" signal. The signal checks if it's running a G with a
preemption request and stops it if so in the same way that stack check
preemptions stop Gs. Since the preemption may fail (the G could be
moved or the signal could arrive at an unsafe point), we keep a count
of the number of received preemption signals. This lets stopG detect
if its request failed and should be retried without an explicit
channel back to suspendG.

For #10958, #24543.

Change-Id: I3e1538d5ea5200aeb434374abb5d5fdc56107e53
Reviewed-on: https://go-review.googlesource.com/c/go/+/201760
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/runtime2.go

@@ -423,6 +423,11 @@ type g struct {
 	preemptStop   bool // transition to _Gpreempted on preemption; otherwise, just deschedule
 	preemptShrink bool // shrink stack at synchronous safe point
 
+	// asyncSafePoint is set if g is stopped at an asynchronous
+	// safe point. This means there are frames on the stack
+	// without precise pointer information.
+	asyncSafePoint bool
+
 	paniconfault bool // panic (instead of crash) on unexpected fault address
 	gcscandone   bool // g has scanned stack; protected by _Gscan bit in status
 	throwsplit   bool // must not split stack
@@ -531,6 +536,11 @@ type m struct {
 	vdsoSP uintptr // SP for traceback while in VDSO call (0 if not in call)
 	vdsoPC uintptr // PC for traceback while in VDSO call
 
+	// preemptGen counts the number of completed preemption
+	// signals. This is used to detect when a preemption is
+	// requested, but fails. Accessed atomically.
+	preemptGen uint32
+
 	dlogPerM
 
 	mOS