[Support] Move getHostNumPhysicalCores to Threading.h

This change is focussed on simplifying `Support/Host.h` to only do
target detection. In this case, this function is close in usage to
existing functions in `Support/Threading.h`, so I moved it into there.
The function is also renamed to `llvm::get_physical_cores()` to match
the style of threading's functions.

Differential Revision: https://reviews.llvm.org/D137836