userland: add assembly support

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

userland/arch/arm/c/add.c

@@ -0,0 +1,17 @@
+/* 1 + 2 == 3 */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint32_t in0 = 1, in1 = 2, out;
+    __asm__ (
+        "add %[out], %[in0], %[in1];"
+        : [out] "=r" (out)
+        : [in0] "r"  (in0),
+          [in1] "r"  (in1)
+    );
+    assert(in0 == 1);
+    assert(in1 == 2);
+    assert(out == 3);
+}