[OpenMP] get logical core count on modern apple platform (#87231)

`hw.logicalcpu` returns the available logical core count

Fix build error for watchOS

```
runtime/src/z_Linux_util.cpp:1821:8: error: 'host_info' is unavailable: not available on watchOS
  rc = host_info(mach_host_self(), HOST_BASIC_INFO, (host_info_t)&info, &num);
       ^
/Applications/Xcode_15.2.app/Contents/Developer/Platforms/WatchOS.platform/Developer/SDKs/WatchOS10.2.sdk/usr/include/mach/mach_host.h:82:15: note: 'host_info' has been explicitly marked unavailable here
kern_return_t host_info
              ^
1 warning and 1 error generated.
make[2]: *** [runtime/src/CMakeFiles/omp.dir/z_Linux_util.cpp.o] Error 1
```