Run make with multiple parallel jobs
Run make with multiple parallel jobs

Verilator runs gcc, which can be parallelized easily and produces great
speedups in compile time.
This patch uses NUMBER_OF_CPUS * 2 parallel jobs (if possible), a
commonly used default value.

lowRISC/fusesoc,lowRISC/fusesoc,olofk/fusesoc,olofk/fusesoc