[sanitizer] Implement GetNumberOfCPUs for Windows

Summary:
Implement `GetNumberOfCPUs` using `GetNativeSystemInfo`.
The only consummer of this function is Scudo which is not functional on
Windows yet.

Reviewers: rnk, zturner

Reviewed By: zturner

Subscribers: zturner, kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42547

llvm-svn: 323462