Compiler: Rename __forceinline define to DOLPHIN_FORCE_INLINE

This is much better as prefixed double underscores are reserved for the
implementation when it comes to identifiers. Another reason its better,
is that, on Windows, where __forceinline is a compiler built-in, with
the previous define, header inclusion software that detects unnecessary
includes will erroneously flag usages of Compiler.h as unnecessary
(despite being necessary on other platforms). So we define a macro
that's used by Windows and other platforms to ensure this doesn't
happen.

Source/Core/Common/BitField.h

@@ -137,7 +137,7 @@ public:
   BitField& operator=(const BitField&) = delete;
 #endif
 
-  __forceinline BitField& operator=(T val)
+  DOLPHIN_FORCE_INLINE BitField& operator=(T val)
   {
     storage = (storage & ~GetMask()) | ((static_cast<StorageType>(val) << position) & GetMask());
     return *this;