enable arm64 tests

The vararg tests had to be changed because
va_list is 32-bit wide on arm.  The astute
reader will notice that the way we pass
va_list values is wrong, we should be using
the ':valist' type as defined below instead
of 'l'.  But eh, that works for now, because
of the ABI.

    type :valist = align 8 { 32 }

test/dark.ssa

@@ -1,3 +1,4 @@
+# skip arm64
 # a hack example,
 # we use a dark type to get
 # a pointer to the stack.