KTL configuration overlay imported from Zircon

Source: zircon/kernel/lib/ktl
Fuchsia revision: f8d670ab4bbb71bfb4d569893ba12fbb8a83bacd

The complete KTL header overlay and its two implementation files, popcount.cc
and stub-new.cc, are imported.  The KTL target is built together with the
Zircon kernel libc header layer and the version-matched libc++ snapshot.