[scudo] Make getNumberOfCPUs Fuchsia compliant v2

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

A first version of this commit was reverted by rL317834 because it broke Android
builds for toolchains generated with older NDKs. This commit introduces a
fall back to solve that issue.

Reviewers: cryptoad, krytarowski, rnk, alekseyshl

Reviewed By: cryptoad, krytarowski, alekseyshl

Subscribers: llvm-commits, srhines, kubamracek, krytarowski

Differential Revision: https://reviews.llvm.org/D40121

llvm-svn: 318802