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/sdktools/vglobals.cpp

@@ -66,7 +66,12 @@ void InitializeValveGlobals()
 		{
 			return;
 		}
+#ifdef PLATFORM_X86
 		g_ppGameRules = *reinterpret_cast<void ***>(addr + offset);
+#else
+		int32_t varOffset = *(int32_t *) ((unsigned char *) addr + offset);
+		g_ppGameRules = *reinterpret_cast<void ***>((unsigned char *) addr + offset + sizeof(int32_t) + varOffset);
+#endif
 	}
 }