runtime: Plan 9, 32-bit: fix build by updating call to open()

With the global redefinition of runtime·open by CL 7543043,
we need to provide a third argument and remove the cast
to the string.

Fixes build on 386 version of Plan 9.

R=khr, rsc, rminnich, ality
CC=golang-dev
https://golang.org/cl/7644047

src/pkg/runtime/time_plan9_386.c

@@ -24,7 +24,7 @@ runtime·nanotime(void)
 	// file descriptor) is roughly four times slower
 	// in 9vx on a 2.16 GHz Intel Core 2 Duo.
 
-	if(fd < 0 && (fd = runtime·open((byte*)"/dev/bintime", OREAD|OCEXEC)) < 0)
+	if(fd < 0 && (fd = runtime·open("/dev/bintime", OREAD|OCEXEC, 0)) < 0)
 		return 0;
 	if(runtime·pread(fd, b, sizeof b, 0) != sizeof b)
 		return 0;