armv6: simplify highvecs enabling code
Use the barrel shifter, Luke.

@@ -84,13 +84,11 @@
         mov     sp, #-1
         cps     #(CPU_MODE_SVC)
 
         /* Enable highvecs (moves the base of the exception vector) */
         mrc     p15, 0, r3, c1, c0, 0
-        mov     r4, #1
-        lsl     r4, r4, #13
-        orr     r3, r3, r4
+        orr     r3, r3, #(1 << 13)
         mcr     p15, 0, r3, c1, c0, 0
 
         /* invoke machine specific setup */
         bl      _mach_start