core: Add Insurgency support for amd64 Windows (#1295)

* Add basic Insurgency support on Windows x64

This allows SourceMod to load on x64 Insurgency. There are still a lot of variable truncation warnings that have to be dealt with.

* Fix 32bit builds

* Compile MySQL extension as well

The hack for __iob_func being removed from the core runtime, but required by the old mysql we're building against can be simplified a lot due to the `_ReturnAddress` intrinsic available since MSVC 2015.

* Don't include the offset we want to extract in the signature

extensions/mysql/mysql/MyDriver.h

@@ -88,6 +88,6 @@ private:
 
 extern MyDriver g_MyDriver;
 
-unsigned int strncopy(char *dest, const char *src, size_t count);
+size_t strncopy(char *dest, const char *src, size_t count);
 
 #endif //_INCLUDE_SM_MYSQL_DRIVER_H_