[OpenMP] get logical core count on modern apple platform

[nihui]

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

[shiltian]

Did you try on macOS? I doubt it will cause any issue but be better on the safe side.

[nihui]

Did you try on macOS? I doubt it will cause any issue but be better on the safe side.

A simple test on github macos x86-64 and M1 machine

https://github.com/nihui/action-protobuf/actions/runs/8524611415/job/23349591784
https://github.com/nihui/action-protobuf/actions/runs/8524611415/job/23349592164

#include <stdio.h>
#include <sys/sysctl.h>
#include <sys/types.h>

int main()
{
    int r = 0;
    size_t len = sizeof(r);
    sysctlbyname("hw.logicalcpu", &r, &len, NULL, 0);
    printf("core count = %d\n", r);
    return 0;
}

[nihui]

Hello, I have no commit access, please commit it for me (smile)