[scudo] Make getNumberOfCPUs Fuchsia compliant

Summary: This change allows Fuchsia to boot properly using the Scudo allocator.

Reviewers: cryptoad, alekseyshl, krytarowski

Reviewed By: cryptoad, krytarowski

Subscribers: krytarowski, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D39490

llvm-svn: 317604