userland: add assembly support

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

userland/arch/arm/address_modes.S

@@ -0,0 +1,51 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#addressing-modes */
+
+#include "common.h"
+
+ENTRY
+
+    /* Offset mode with immediate. Add 4 to the address register, which ends up
+     * reading myvar2 instead of myvar.
+     */
+    adr r0, myvar
+    ldr r1, [r0, 4]
+    ASSERT_EQ(r1, 0x9ABCDEF0)
+    /* r0 was not modified. */
+    ASSERT_EQ(r0, myvar)
+
+    /* Pre-indexed mode */
+    adr r0, myvar
+    ldr r1, [r0, 4]!
+    ASSERT_EQ(r1, 0x9ABCDEF0)
+    /* r0 was modified. */
+    ASSERT_EQ(r0, myvar2)
+
+    /* Post-indexed mode */
+    adr r0, myvar
+    ldr r1, [r0], 4
+    ASSERT_EQ(r1, 0x12345678)
+    /* r0 was modified. */
+    ASSERT_EQ(r0, myvar2)
+
+    /* Offset in register. */
+    adr r0, myvar
+    mov r1, 4
+    ldr r2, [r0, r1]
+    ASSERT_EQ(r2, 0x9ABCDEF0)
+
+    /* Offset in shifted register:
+     * r2 =
+     * (r0 + (r1 << 1))
+     * == *(myvar + (2 << 1))
+     * == *(myvar + 4)
+     */
+    adr r0, myvar
+    mov r1, 2
+    ldr r2, [r0, r1, lsl 1]
+    ASSERT_EQ(r2, 0x9ABCDEF0)
+
+EXIT
+myvar:
+    .word 0x12345678
+myvar2:
+    .word 0x9ABCDEF0