[LibOS] Fix R_OK and X_OK macros for file permissions

These macros were mixed up. However, they were used only in the syscall
parser so this mix up didn't lead to any functionality issues. Noticed
when analyzing Gramine logs; kudos to Jinen Gandhi and Anjali Rai.

Signed-off-by: Dmitrii Kuvaiskii <dmitrii.kuvaiskii@intel.com>

libos/include/libos_fs.h

@@ -504,10 +504,10 @@ struct libos_mount {
 
 extern struct libos_dentry* g_dentry_root;
 
-#define F_OK 0
-#define R_OK        001
-#define W_OK        002
-#define X_OK        004
+#define F_OK   0
+#define X_OK 001
+#define W_OK 002
+#define R_OK 004
 #define MAY_EXEC  001
 #define MAY_WRITE 002
 #define MAY_READ  004