runtime: fix netbsd/arm build

Currently fails with:
fatal error: runtime: stack split during syscall
goroutine 2 [stack split]:
_vasop(0x3ac4a0, 0x505f8f00, 0x7a5a8, 0x7, 0x1ed3797f, ...)
        src/pkg/runtime/vlrt_arm.c:513 fp=0x505f8ecc
runtime.semasleep(0xf8475800, 0xd)
        src/pkg/runtime/os_netbsd.c:97 +0x178 fp=0x505f8efc

R=rsc
CC=golang-dev
https://golang.org/cl/12246043

src/pkg/runtime/os_netbsd.c

@@ -94,7 +94,7 @@ runtime·semasleep(int64 ns)
 				runtime·atomicstore(&m->waitsemalock, 0);
 				runtime·lwp_park(nil, 0, &m->waitsemacount, nil);
 			} else {
-				ns += runtime·nanotime();
+				ns = ns + runtime·nanotime();
 				// NOTE: tv_nsec is int64 on amd64, so this assumes a little-endian system.
 				ts.tv_nsec = 0;
 				ts.tv_sec = runtime·timediv(ns, 1000000000, (int32*)&ts.tv_nsec);