Set parallel builds to use number of physical cores

The ProcessorCount module returns the number of logical cores, which may use too many resources and lock up the machine. This sets the parallelism for building and testing using CMake's builtin variables and uses only the total number of physical cores to avoid using too many resources at once.

Note: The builds using Clang 13.0 and native instructions are locking up in Jenkins, likely due to using too many threads.