userland: add assembly support

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

userland/arch/common.h

@@ -0,0 +1,28 @@
+#ifndef COMMON_H
+#define COMMON_H
+
+/* We define in this header only macros that are the same on all archs. */
+
+/* common_arch.h contains arch specific macros. */
+#include "common_arch.h"
+
+.extern \
+    exit, \
+    printf, \
+    puts \
+;
+
+/* Assert that the given branch instruction is taken. */
+#define ASSERT(branch_if_pass) \
+    branch_if_pass 1f; \
+    FAIL; \
+1: \
+;
+
+/* Assert that a register equals another register. */
+#define ASSERT_EQ_REG(reg1, reg2) \
+	cmp reg1, reg2; \
+	ASSERT(beq); \
+;
+
+#endif