Revert "Host: Internalize computeHostNumPhysicalCores/computeHostNumHardwareThreads"

This reverts commit 9969ceb3.

On Windows:

lld-link: error: undefined symbol: int __cdecl computeHostNumPhysicalCores(void)
>>> referenced by LLVMSupport.lib(Support.Host.obj):(int __cdecl llvm::sys::getHostNumPhysicalCores(void))