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/VideoCommon/IndexGenerator.cpp

@@ -58,7 +58,8 @@ void IndexGenerator::AddIndices(int primitive, u32 numVerts)
 
 // Triangles
 template <bool pr>
-__forceinline u16* IndexGenerator::WriteTriangle(u16* Iptr, u32 index1, u32 index2, u32 index3)
+DOLPHIN_FORCE_INLINE u16* IndexGenerator::WriteTriangle(u16* Iptr, u32 index1, u32 index2,
+                                                        u32 index3)
 {
   *Iptr++ = index1;
   *Iptr++ = index2;