Remove net.link.ether.inet.useloopback sysctl tunable. It was always on by
default from the very beginning. It was placed in wrong namespace
net.link.ether, originally it had been at another wrong namespace. It was
incorrectly documented at incorrect manual page arp(8). Since new-ARP commit,
the tunable have been consulted only on route addition, and ignored on route
deletion. Behaviour of a system with tunable turned off is not fully correct,
and has no advantages comparing to normal behavior.
In complemence to ifa_add_loopback_route() and ifa_del_loopback_route()
provide function ifa_switch_loopback_route() that will be used in case when
an interface address used for a loopback route goes away, but we have another
interface address with same address value and want to preserve loopback
route.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix libgcc build with gcc after r257645, by using -Wno-static-in-inline
for clang only.