CodeBlock: Add a shared IsAlmostFull function

This function shall keep care about the low watermark of code space.
If we ran out of space, the JITs shall clear their block cache.

Source/Core/Core/PowerPC/Jit64IL/JitIL.cpp

@@ -473,7 +473,7 @@ void JitIL::Trace()
 
 void JitIL::Jit(u32 em_address)
 {
-	if (GetSpaceLeft() < 0x10000 || farcode.GetSpaceLeft() < 0x10000 || blocks.IsFull() ||
+	if (IsAlmostFull() || farcode.IsAlmostFull() || blocks.IsFull() ||
 		SConfig::GetInstance().bJITNoBlockCache)
 	{
 		ClearCache();