Add the ability to link programs against a compat ABI.

Linkage is controlled by two make knobs:
WANT_COMPAT - Prefer to link against the compat ABI.
NEED_COMPAT - Link against the compat ABI or fail to build.

Supported values are "32", "soft", and "any". The latter meaning pick
the first[0] supported compat ABI.

This can be used to provide test binaries for compat ABIs or to link
ABI-specific programs.

[0] We currently support only one compat ABI at a time, but this may
change in the future and some code in this commit is structured to ease
that change.

Reviewed by: bdrewery, jhb
Obtained from: CheriBSD (in concept)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22023