[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/kernel/xboxkrnl/xboxkrnl_strings.cc

@@ -15,7 +15,6 @@
 #include "xenia/kernel/kernel_state.h"
 #include "xenia/kernel/user_module.h"
 #include "xenia/kernel/util/shim_utils.h"
-#include "xenia/kernel/util/xex2.h"
 #include "xenia/kernel/xboxkrnl/xboxkrnl_private.h"
 #include "xenia/kernel/xthread.h"
 #include "xenia/xbox.h"