runtime: mark usleep as NOSPLIT on Plan 9

Usleep is called from lockextra, also marked NOSPLIT.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9258043

src/pkg/runtime/os_plan9.c

@@ -122,6 +122,7 @@ runtime·osyield(void)
 	runtime·sleep(0);
 }
 
+#pragma textflag 7
 void
 runtime·usleep(uint32 µs)
 {