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/arm/push.S

@@ -5,27 +5,27 @@
 ENTRY
 
     /* Save sp before push. */
-    mov r0, sp
+    mov r4, sp
 
     /* Push. */
-    mov r1, 1
-    mov r2, 2
-    push {r1, r2}
+    mov r5, 1
+    mov r6, 2
+    push {r5, r6}
 
     /* Save sp after push. */
-    mov r1, sp
+    mov r5, sp
 
     /* Restore. */
-    mov r3, 0
-    mov r4, 0
-    pop {r3, r4}
-    ASSERT_EQ(r3, 1)
-    ASSERT_EQ(r4, 2)
+    mov r7, 0
+    mov r8, 0
+    pop {r7, r8}
+    ASSERT_EQ(r7, 1)
+    ASSERT_EQ(r8, 2)
 
     /* Check that stack pointer moved down by 8 bytes
      * (2 registers x 4 bytes each).
      */
-    sub r0, r1
-    ASSERT_EQ(r0, 8)
+    sub r4, r5
+    ASSERT_EQ(r4, 8)
 
 EXIT