runtime: improve mmap return value checking for netbsd/openbsd

Rather than just checking for ENOMEM, check for a return value of less
than 4096, so that we catch other errors such as EACCES and EINVAL.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7942043

src/pkg/runtime/mem_netbsd.c

@@ -50,7 +50,7 @@ runtime·SysReserve(void *v, uintptr n)
 		return v;
 
 	p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-	if(p == (void*)ENOMEM)
+	if(p < (void*)4096)
 		return nil;
 	return p;
 }

src/pkg/runtime/mem_openbsd.c

@@ -50,7 +50,7 @@ runtime·SysReserve(void *v, uintptr n)
 		return v;
 
 	p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-	if(p == (void*)ENOMEM)
+	if(p < (void*)4096)
 		return nil;
 	return p;
 }