[scudo] Correct performance regression in Secondary

Summary:
This wasn't noticed: `RoundUpTo` doesn't produce a constant expression, so the
sizes were not constant either. Enforce them to be static const, replace
`RoundUpTo` by its expression. The compiler can now optimize the associated
computations accordingly.

Also looking at the produced assembly, `PageSize` was fetched multiple times
during `Allocate`, so keep a local value of it. As a result it's fetched once
and kept in a register.

Reviewers: alekseyshl, flowerhack

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40862

llvm-svn: 319903