runtime: mark some C globals as having no pointers.

C globals are conservatively scanned.  This helps
avoid false retention, especially for 32 bit.

LGTM=rsc
R=golang-codereviews, khr, rsc
CC=golang-codereviews
https://golang.org/cl/102040043

src/pkg/runtime/os_openbsd.c

@@ -167,6 +167,7 @@ runtime·osinit(void)
 void
 runtime·get_random_data(byte **rnd, int32 *rnd_len)
 {
+	#pragma dataflag NOPTR
 	static byte urandom_data[HashRandomBytes];
 	int32 fd;
 	fd = runtime·open("/dev/urandom", 0 /* O_RDONLY */, 0);