Common: Add header for compiler-specifics

Instead of globbing things under an ambiguous Common.h header, move
compiler-specifics over to Compiler.h. This gives us a dedicated home
for anything related to compilers that we want to make functional across
all compilers that we support.

This moves us a little closer to eliminating Common.h entirely.

Source/Core/Core/HW/WiimoteEmu/Attachment/Attachment.cpp

@@ -8,8 +8,8 @@
 #include <array>
 #include <cstring>
 
-#include "Common/Common.h"
 #include "Common/CommonTypes.h"
+#include "Common/Compiler.h"
 #include "Core/HW/WiimoteEmu/WiimoteEmu.h"
 #include "InputCommon/ControllerEmu/ControlGroup/Extension.h"