parsers: dynamically register parsers

Currently, we require all parsers to be defined in an array in
parsers.c.

This change removes this requirement, by introducting a
register_parser() macro, which adds a constructor to register the parser
with the core parser infrastructure.

Because each parser no longer resolves an undefined symbol, we need to
use a `ld -r` object for libparser, instead of using libtool, which
creates a .a (and hence has no parsers included).

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>