System.CPUCount is not implemented for MacOS and returns always 1
## Summary
<!--Summarize the wanted feature concisely-->
## System Information
<In MacOS/Darwin the function System.CPUCount always return one. Suggest to either add a compiler warning hint "system.CPUcount is not implemented and returns always 1" or much better implement in RTL. Use code from this 2008 unit:
function GetSystemThreadCount in
lazarus/components/multithreadprocs/mtpcpu.pas
Note System.CPUCount is for Linux implemented in trunk.
>
- **Operating system:**
MacOS
- **Processor architecture:**
X86-64, M processors
- **Device:**
Computer
## Example Project
<lazarus/components/multithreadprocs/mtpcpu.pas>
## Relevant 3rd party information
<>
issue