[libFuzzer] Properly use unsigned for workers, jobs and NumberOfCpuCores.

std::thread::hardware_concurrency() returns an unsigned, so I modify
NumberOfCpuCores() to return unsigned too.
The number of cpus is used to define the number of workers, so I decided
to update the worker and jobs flags to be declared as unsigned too.

Differential Revision: https://reviews.llvm.org/D27685

llvm-svn: 289559