abd_os: break out platform-specific header parts

Removing the platform #ifdefs from shared headers in favour of
per-platform headers. Makes abd_t much leaner, among other things.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16253