Make our architecture defines less stupid.

Our defines were never clear between what meant 64bit or x86_64
This makes a clear cut between bitness and architecture.
This commit also has the side effect of bringing up aarch64 compiling support.

Source/Core/VideoBackends/Software/Rasterizer.cpp

@@ -86,17 +86,9 @@ inline int iround(float x)
 {
 	int t;
 
-#if defined(_WIN32) && !defined(_M_X64)
-	__asm
-	{
-		fld  x
-		fistp t
-	}
-#else
 	t = (int)x;
 	if((x - t) >= 0.5)
 		return t + 1;
-#endif
 
 	return t;
 }