fix sysctlt compilation error on FreeBSD

[wfurt]

This is regression caused by #27048 and build fails like:

[ 28%] Built target utilcodestaticnohost
/home/furt/git/coreclr/src/pal/src/misc/sysinfo.cpp:382:10: error: use of undeclared identifier 'sysctlnametomib'
    rc = sysctlnametomib("vm.swap_info", mib, &length);
         ^
[ 35%] Built target cee_dac
/home/furt/git/coreclr/src/pal/src/misc/sysinfo.cpp:390:18: error: use of undeclared identifier 'sysctl'
            rc = sysctl(mib, 3, &xsw, &length, NULL, 0);
                 ^
/home/furt/git/coreclr/src/pal/src/misc/sysinfo.cpp:599:30: error: use of undeclared identifier 'sysctlbyname'
        const bool success = sysctlbyname("hw.l3cachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0
                             ^
/home/furt/git/coreclr/src/pal/src/misc/sysinfo.cpp:600:16: error: use of undeclared identifier 'sysctlbyname'
            || sysctlbyname("hw.l2cachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0
               ^
/home/furt/git/coreclr/src/pal/src/misc/sysinfo.cpp:601:16: error: use of undeclared identifier 'sysctlbyname'
            || sysctlbyname("hw.l1dcachesize", &cacheSizeFromSysctl, &sz, nullptr, 0) == 0;
[ 35%] Generating eventpipe/eventpipehelpers.cpp, eventpipe/dotnetruntime.cpp, eventpipe/dotnetruntimerundown.cpp, eventpipe/dotnetruntimestress.cpp, eventpipe/dotnetruntimeprivate.cpp

The problem is that FreeBSD has HAVE_SYSCONF and that seems mutually elusive in includes but later on used with different guard. Maybe it would be better to set HAVE_SYSCONF=0 on Linux if we want to avoid usage of it.

This is minimal change to fix compilation while preserving spirit of #27048.
Note that FreeBSD does not have vm.swap_info. We will probably need separate effort to find some equivalent.

cc: @omajid

[omajid]

@wfurt Thanks for fixing this. Should we add FreeBSD (and/or other BSDs) to our CI system?

[wfurt]

@wfurt Thanks for fixing this. Should we add FreeBSD (and/or other BSDs) to our CI system?

That is long story.
FreeBSD used to be there but unfortunately it was removed as there is no official Azure or .NET support.

[janvorli]

Can you please use ! instead of not here and below?

[wfurt]

updated.