[CPU] Move XEX2 code into XexModule class, autodetect XEX key

Code is mainly just copy/pasted from kernel/util/xex2.cc, I've tried fixing it up to work better in a class, but there's probably some things I missed.

Also includes some minor improvements to the XEX loader, like being able to try both XEX keys (retail/devkit) automatically, and some fixes to how the base address is determined.

(Previously there was code that would get base address from optional header, code that'd get it from xex_security_info, code that'd use a stored base address value...
Now everything reads it from a single stored value instead, which is set either from the xex_security_info, or if it exists from the optional header instead.
Maybe this can help improve compatibility with any weird XEX's that don't have a base address optional header?)

Compressed XEX loader also has some extra checks to make sure the compressed data hash matches what's expected.
Might increase loading times by a fraction, but could save reports from people unknowingly using corrupt XEXs.
(still no checks for non-compressed data though, maybe need to compare data with xex_security_info->ImageHash?)

src/xenia/base/byte_order.h

@@ -69,6 +69,8 @@ template <typename T>
 inline T byte_swap(T value) {
   if (sizeof(T) == 4) {
     return static_cast<T>(byte_swap(static_cast<uint32_t>(value)));
+  } else if (sizeof(T) == 2) {
+    return static_cast<T>(byte_swap(static_cast<uint16_t>(value)));
   } else {
     assert_always("not handled");
   }