runtime: never allocate new M when jumping time forward

When we're jumping time forward, it means everyone is asleep, so there
should always be an M available. Furthermore, this causes both
allocation and write barriers in contexts that may be running without
a P (such as in sysmon).

Hence, replace this allocation with a throw.

Updates #10600.

Change-Id: I2cee70d5db828d0044082878995949edb25dda5f
Reviewed-on: https://go-review.googlesource.com/16815Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc.go

@@ -3274,11 +3274,12 @@ func checkdead() {
 		}
 		mp := mget()
 		if mp == nil {
-			newm(nil, _p_)
-		} else {
-			mp.nextp.set(_p_)
-			notewakeup(&mp.park)
+			// There should always be a free M since
+			// nothing is running.
+			throw("checkdead: no m for timer")
 		}
+		mp.nextp.set(_p_)
+		notewakeup(&mp.park)
 		return
 	}