userland: add assembly support

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

userland/arch/arm/hello_driver.S

@@ -0,0 +1,23 @@
+/* Minimal example using driver.
+ *
+ * Controls the exit status of the program.
+ */
+
+.syntax unified
+.text
+.global asm_main
+asm_main:
+asm_main_after_prologue:
+
+    /* Set the return value according to the ARM calling convention. */
+    mov r0, 0
+
+    /* Try some whacky value to see tests break. */
+    /*mov r0, 77*/
+
+    /* Branch to the address at register lr.
+     * That is the return value which was put there by the C driver (likely with a bl).
+     *
+     * X means eXchange encoding from thumb back to ARM, which is what the driver uses.
+     */
+    bx lr