[CPU] Add XEXP support to XexModule, if XEXP is in same folder as XEX

This was a headache to work out, big thanks to the lack of documentation on .xexp files... a ton of guesswork was involved here but luckily it turned out well.

I did have to make some pretty major changes to the way XEX files are loaded though.
Previously it'd just load everything in one go: XEX headers -> decrypt/decompress data -> load imports/symbols -> set loader data table entries, etc...

Now it's changed to something like this:
- Load base XEX headers + decrypted/decompressed image data, return X_STATUS_PENDING
- In the LoadFromFile call used to load the XEX, search for XEXP patch file (only .xexp in same folder atm)
- If patch exists: load XEXP, decrypt headers/data, apply patch to base XEX, dispose of XEXP
- Finish XEX load via LoadXexContinue() (handles imports/symbols/loader data...)

This saves us from needing to reset the imports/function/symbol stuff after patching (since all the XEX code will be a lot different), but I'm not really sure if I went about it the best way.

src/xenia/kernel/user_module.h

@@ -92,6 +92,8 @@ class UserModule : public XModule {
                                         ByteStream* stream, std::string path);
 
  private:
+  X_STATUS LoadXexContinue();
+
   uint32_t guest_xex_header_ = 0;
   ModuleFormat module_format_ = kModuleFormatUndefined;