Revert "[Support] Move getHostNumPhysicalCores to Threading.h"

This reverts commit 5577207d.

This breaks building LLVM on recent macOS. Error messages below:

llvm/lib/Support/Threading.cpp:190:3: error: use of undeclared
identifier 'sysctlbyname'
  sysctlbyname("hw.physicalcpu", &count, &len, NULL, 0);
    ^

llvm/lib/Support/Threading.cpp:193:13: error: use of undeclared
identifier 'CTL_HW'
    nm[0] = CTL_HW;
            ^

llvm/lib/Support/Threading.cpp:194:13: error: use of undeclared identifier 'HW_AVAILCPU'
    nm[1] = HW_AVAILCPU;
            ^

llvm/lib/Support/Threading.cpp:195:5: error: use of undeclared identifier 'sysctl'
    sysctl(nm, 2, &count, &len, NULL, 0);
    ^