Refactor ptrace() ABI compatibility.

Add a freebsd32_ptrace() and move as many freebsd32 shims as possible
to freebsd32_ptrace(). Aside from register sets, freebsd32 passes
pointers to native structures to kern_ptrace() and converts to/from
native/32-bit structure formats in freebsd32_ptrace() outside of
kern_ptrace().

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D25195