userland: add assembly support

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

userland/arch/arm/c/inc_memory_global.c

@@ -0,0 +1,25 @@
+/* GCC 8.2.0 -O0 and -O3 assembles ldr line to:
+ *
+ * ....
+ * movw r3, #<lower address part>
+ * movt r3, #<higher address part>
+ * ldr r0, [r3]
+ * ....
+ */
+
+#include <assert.h>
+#include <inttypes.h>
+
+uint32_t my_global_var = 1;
+
+int main(void) {
+    __asm__ (
+        "ldr r0, %[my_global_var];"
+        "add r0, r0, #1;"
+        "str r0, %[my_global_var];"
+        : [my_global_var] "+m" (my_global_var)
+        :
+        : "r0"
+    );
+    assert(my_global_var == 2);
+}