Make cpp mac target log some system stats

Based on discussion in https://github.com/larsiusprime/crashdumper/issues/10

Currently sses command`system_profiler SPHardwareDataType -detailLevel mini` something like four times, which might be a bit excessive. The call count can be reduced by combining os.sh, memory.sh and cpu.sh and then parsing the combined output a bit differently in SystemData.hx.
Store memory in KB on mac target as well
Merge pull request #12 from leolannenmaki/master

Make cpp mac target log some system stats