userland: add assembly support

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

userland/arch/arm/c/inc.c

@@ -0,0 +1,15 @@
+/* Increment a variable in inline assembly. */
+
+#include <assert.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint32_t my_local_var = 1;
+    __asm__ (
+        "add %[my_local_var], %[my_local_var], #1;"
+        : [my_local_var] "+r" (my_local_var)
+        :
+        :
+    );
+    assert(my_local_var == 2);
+}