cdefs: Add __deprecated

Add __deprecated decorator. This is for a deprecated interface. copystr
is tagged with this today in copy(9), but don't actually provide it or
use it. copystr is a #define so adding it will have to wait.

LinuxKPI was defining this away completely in compiler.h. Since this is
shared between Linux KPI consumers and OpenZFS, if it's already defined,
use the FreeBSD sys/cdefs.h version, otherwise define it away. For
OpenZFS this will retain it, while for Linux KPI it will tend to drop it
(I think always, but I didn't look at everything).

Sponsored by: Netflix
Reviewed by: jhb, emaste
Differential Revision: https://reviews.freebsd.org/D46137