[scudo] Allow for compile-time choice of the SizeClassMap

Summary:
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!

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: llvm-commits, srhines

Differential Revision: https://reviews.llvm.org/D40521

llvm-svn: 319350