Implement GetCurrentProcessCpuCount for Unix and merge NumberOfProcesors implementations. (#11742)
* Implement GetCurrentProcessCpuCount for Unix and merge NumberOfProcessors implementations.
* Split GetCurrentProcessCpuCount in PAL and non-PAL parts
* gcenv.unix.cpp: GetCurrentProcessAffinityMask: revert, GetCurrentProcessCpuCount: add implementation
* bugfix: #if instead of #ifdef HAVE_SCHED_GETAFFINITY
* Remove PAL_GetCurrentProcessCpuCount
* gcenv.unix: implement GetCurrentProcessAffinityMask, GetCurrentProcessCpuCount
* numa.cpp: initialize g_cpuToAffinity[i].Number when numa.h is missing
* cleanup
* mscorwks_unixexports.src: Add GetProcessAffinityMask
* PR feedback
* Add GetProcessAffinityMask to mscordac_unixexports.src