userland: add assembly support

Move arm assembly cheat here, and start some work on x86 cheat as well.

userland/arch/arm/inc_array.S

@@ -0,0 +1,27 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#loop-over-array */
+
+#include "common.h"
+
+#define NELEM 4
+#define ELEM_SIZE 4
+
+.data;
+my_array:
+    .word 0x11111111, 0x22222222, 0x33333333, 0x44444444
+my_array_expect:
+    .word 0x11111112, 0x22222223, 0x33333334, 0x44444445
+
+ENTRY
+    /* Increment. */
+    ldr r0, =my_array
+    mov r1, NELEM
+increment:
+    ldr r2, [r0]
+    add r2, 1
+    /* Post index usage. */
+    str r2, [r0], ELEM_SIZE
+    sub r1, 1
+    cmp r1, 0
+    bne increment
+    ASSERT_MEMCMP(my_array, my_array_expect, 0x10)
+EXIT