Introduce an AUXARGS_ENTRY_PTR() macro.

As the name implys, it uses the a_ptr member of the auxarg entry (except
in compat32 where it uses a_val). This is more correct and required for
systems where a_val is not the same size or hardware type as a_ptr (e.g.
CHERI).

This is a prepratory commit for D24407.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA