[sanitizer] Revert rL317822

Summary:
This reverts D39490.

For toolchains generated with older NDKs (<=r13b as far as we tested),
`cpu_set_t` doesn't exist in `sched.h`.
We have to figure out another way to get the number of CPUs without this.

Reviewers: rnk

Reviewed By: rnk

Subscribers: kubamracek, llvm-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D39867

llvm-svn: 317834