With this change, we allow someone to chose the SizeClassMap they want to use
at compile time via a define.
I feel somewhat unimaginative with the name of the defines, so if someone has a
better idea, let me know. I have been alternating between those and
SCUDO_USE_xxx_SIZECLASSMAP which is clearer but also longer. The issue with
those is that it wouldn't be consistent with SCUDO_TSD_EXCLUSIVE that should
probably become SCUDO_USE_EXCLUSIVE_TSD maybe?
Anyway, naming is hard, and I am not sure what makes more sense!
Why not just fold it into #else?