runtime: let freezetheworld work even when gomaxprocs=1

Freezetheworld still has stuff to do when gomaxprocs=1.
In particular, signals can come in on other Ms (like the GC M, say)
and the single user M is still running.

Fixes #10546

Change-Id: I2f07f17d1c81e93cf905df2cb087112d436ca7e7
Reviewed-on: https://go-review.googlesource.com/9551Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/runtime/proc1.go

@@ -216,9 +216,6 @@ const freezeStopWait = 0x7fffffff
 // There is no reverse operation, used during crashing.
 // This function must not lock any mutexes.
 func freezetheworld() {
-	if gomaxprocs == 1 {
-		return
-	}
 	// stopwait and preemption requests can be lost
 	// due to races with concurrently executing threads,
 	// so try several times