Saving all registers in syscall so that return from fork is easier.

Child needs rewound function stack in order to reach registers r9-r12
that have original userspace values. But we jump to return_from_syscall
without rewinding the stack. Therefore to ease context restore, we save
r9-r12 on the stack as well upon syscall entry.

include/l4/glue/arm/syscall.h

@@ -33,6 +33,10 @@ typedef struct syscall_context {
 	u32 r6;		/* MR3 */
 	u32 r7;		/* MR4 */
 	u32 r8;		/* MR5 */
+	u32 r9;
+	u32 r10;
+	u32 r11;
+	u32 r12;
 	u32 sp_usr;
 	u32 lr_usr;
 } __attribute__((__packed__)) syscall_context_t;