userland: add assembly support

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

userland/arch/arm/push.S

@@ -0,0 +1,31 @@
+/* https://github.com/cirosantilli/arm-assembly-cheat#ldmia */
+
+#include "common.h"
+
+ENTRY
+
+    /* Save sp before push. */
+    mov r0, sp
+
+    /* Push. */
+    mov r1, 1
+    mov r2, 2
+    push {r1, r2}
+
+    /* Save sp after push. */
+    mov r1, sp
+
+    /* Restore. */
+    mov r3, 0
+    mov r4, 0
+    pop {r3, r4}
+    ASSERT_EQ(r3, 1)
+    ASSERT_EQ(r4, 2)
+
+    /* Check that stack pointer moved down by 8 bytes
+     * (2 registers x 4 bytes each).
+     */
+    sub r0, r1
+    ASSERT_EQ(r0, 8)
+
+EXIT