[scudo] Allow to specify the maximum number of TSDs at compile time

Summary:
This introduces `SCUDO_MAX_CACHES` allowing to define an upper bound to the
number of `ScudoTSD` created in the Shared TSD model (by default 32U).
This name felt clearer than `SCUDO_MAX_TSDS` which is technically what it really
is. I am opened to suggestions if that doesn't feel right.

Additionally change `getNumberOfCPUs` to return a `u32` to be more consistent.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39338

llvm-svn: 316788