netlink: add NETLINK to the DEFAULTS for each architecture

NETLINK is going to replace rtsock and a number of other ioctl/sysctl interfaces.
In-base utilies such as route(8), netstat(8) and soon ifconfig(8)
are being converted to use netlink sockets as a transport between
kernel and userland.
In the current configuration, it still possible have the kernel
without NETLINK (nooptions NETLINK) and use the aforementioned
utilies by buidling the world with WITHOUT_NETLINK src.conf knob.
However, this approach does not cover the cases when person unintentionally
builds a custom kernel without netlink and tries to use the standard userland.

This change adds option NETLINK to the default options for each
architecture, fixing the custom kernel issue.
For arm, this change uses std.armv6 and std.armv7 (netlink already in)
instead of DEFAULTS.

Reviewed By: imp
Differential Revision: https://reviews.freebsd.org/D39339