userland: add assembly support

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

userland/arch/arm/add.S

@@ -0,0 +1,58 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#data-processing-instructions */
+
+#include "common.h"
+
+ENTRY
+
+    /* Immediate encoding.
+     *
+     * r1 = r0 + 2
+     */
+    mov r0, 1
+    /* r1 = r0 + 2 */
+    add r1, r0, 2
+    ASSERT_EQ(r1, 3)
+
+    /* If src == dest, we can omit one of them.
+     *
+     * r0 = r0 + 2
+     */
+    mov r0, 1
+    add r0, 2
+    ASSERT_EQ(r0, 3)
+
+    /* Same as above but explicit. */
+    mov r0, 1
+    add r0, r0, 2
+    ASSERT_EQ(r0, 3)
+
+#if 0
+    /* But we cannot omit the register if there is a shift when using .syntx unified:
+     * https://github.com/cirosantilli/arm-assembly-cheat#shift-suffixes
+     */
+    .syntax unified
+    /* Error: garbage following instruction */
+    add r0, r1, lsl 1
+    /* OK */
+    add r0, r0, r1, lsl 1
+#endif
+
+    /* Register encoding.
+     *
+     * r2 = r0 + r1
+     */
+    mov r0, 1
+    mov r1, 2
+    add r2, r0, r1
+    ASSERT_EQ(r2, 3)
+
+    /* Register encoding, omit implicit register.
+     *
+     * r1 = r1 + r0
+     */
+    mov r0, 1
+    mov r1, 2
+    add r1, r0
+    ASSERT_EQ(r1, 3)
+
+EXIT