optimize: Add GetTimeSinceStartFast() that caches the time for archs where GetMicrosecondsSinceStart() is slow

stepmania/src/BGAnimationLayer.cpp

@@ -627,7 +627,7 @@ void BGAnimationLayer::Update( float fDeltaTime )
 		break;
 	case TYPE_TILES:
 		{
-			float fSecs = RageTimer::GetTimeSinceStart();
+			float fSecs = RageTimer::GetTimeSinceStartFast();
 			float fTotalWidth = m_iNumTilesWide * m_fTilesSpacingX;
 			float fTotalHeight = m_iNumTilesHigh * m_fTilesSpacingY;