Build cores in parellel

[garbear]

Description

This PR modifies the core's CMakeLists.txt file to add the parallel -j flag to the build command.

Credit to @fuzzard.

How has this been tested?

Ran locally on my workstation:

[ 29%] Performing configure step for 'scummvm'
-- The C compiler identification is GNU 11.4.0
-- The CXX compiler identification is GNU 11.4.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using 16 threads deduced by ProcessorCount
-- Configuring done
-- Generating done

• "Using 16 threads deduced by ProcessorCount" appears in the output.

Ran on ScummVM. Jenkins results: https://jenkins.kodi.tv/blue/organizations/jenkins/kodi-game%2Fgame.libretro.scummvm/detail/2.9.0.44-Omega/1/pipeline