Use STD*_FILENO constants (#1055)

These are defined in platform.h, but not consistently used

Co-authored-by: Eldred Habert <eldredhabert0@gmail.com>

src/asm/output.c

@@ -492,7 +492,7 @@ void out_WriteObject(void)
 	if (strcmp(objectName, "-") != 0)
 		f = fopen(objectName, "wb");
 	else
-		f = fdopen(1, "wb");
+		f = fdopen(STDOUT_FILENO, "wb");
 
 	if (!f)
 		err("Couldn't write file '%s'", objectName);