freebsd: simplify MD isa_defs.h

Most of this file was a pile of defines, apparently from Solaris that
controlled nothing in the source tree. A few things controlled the
definition of unused types or macros which I have removed.

Considerable further cleanup is possible including removal of
architectures FreeBSD never supported. This file should likely converge
with the Linux version to the extent possible.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes #14127