runtime: disable preemption during test calls to futexsleep

Fixes #35347

Change-Id: If7380f29e97a5abe86cdd5e2853323de7997ccfc
Reviewed-on: https://go-review.googlesource.com/c/go/+/205378
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/export_futex_test.go

@@ -6,5 +6,14 @@
 
 package runtime
 
-var Futexsleep = futexsleep
 var Futexwakeup = futexwakeup
+
+//go:nosplit
+func Futexsleep(addr *uint32, val uint32, ns int64) {
+	// Temporarily disable preemption so that a preemption signal
+	// doesn't interrupt the system call.
+	poff := debug.asyncpreemptoff
+	debug.asyncpreemptoff = 1
+	futexsleep(addr, val, ns)
+	debug.asyncpreemptoff = poff
+}