userland: start refactor to show failing values on failure!

aarch64 basically done, but missing:

- other archs
- maybe convert main.c into C++ to use templates?
- full review of ASSERT_EQ calling convention issues not seen by tests
  by chance
- documentation

userland/arch/common.h

@@ -21,10 +21,12 @@
 1: \
 ;
 
+#ifndef ASSERT_EQ_REG
 /* Assert that a register equals another register. */
 #define ASSERT_EQ_REG(reg1, reg2) \
     cmp reg1, reg2; \
     ASSERT(beq); \
 ;
+#endif
 
 #endif