Add XCELIUM to existing IUS define checks.

[cmarqu]

For some reason I missed to also check for the XCELIUM define when I introduced Makefile.xcelium, fix that.

[themperek]

Is not better to just keep one in the define (across the code)?

[cmarqu]

I thought about this but didn't like it so much - it then has some magic going on behind the scenes, instead of the current straightforward simname.upper() == DEFINE relation.
Also, I would expect that in the future with ghdl and Verilator getting incrementally more VPI/VHPI support, we may need to have more such switches based on both simulator names and also their versions, so the slightly more complex #if defined will not stand out much.

Of course, I'm open to change this if normalizing to a single define is the mainstream opinion.

[eric-wieser]

The normalization could happen in the c preprocessor. Is there a name that covers both of these?

[cmarqu]

CADENCE would cover both.

[themperek]

For Questa we keep MODELSIM do not see why we cannot do the same here.

[cmarqu]

Alright, I now used the method of the existing ModelSim/Questa normalization.

[eric-wieser]

Not sure this is the cleanest approach, but it's what we already do, and consistency is probably more important