comp: 'option extra_link_args' needs a string

.. which was formerly not accepted in the grammar, making
the option useless.

Signed-off-by: Jeff Epler <jepler@unpythonic.net>
comp: test that option extra_link_args works

Signed-off-by: Jeff Epler <jepler@unpythonic.net>
comp: document halcompile extra_link_args

Signed-off-by: Jeff Epler <jepler@unpythonic.net>