sysent: move away from allowing all compat options for other ABIs

Notably, the current compat_options only makes sense for native and
freebsd32 ABIs. For the others, it just adds cruft. Switch to having
sets of compat options, and default to the native set. Setup the other
ABIs where it doesn't make sense to opt-out of the native set.

This removes some redundant COMPAT_FREEBSD* stuff from Linuxolator bits.

line_expr in makesyscalls.lua is fixed to allow empty strings to be
specified, since they're harmless.

Reviewed by: brooks, kib (both earlier version)
Differential Revision: https://reviews.freebsd.org/D33356