Fix iOS builds with assembly.

The patch in 76e831d9 got us most of the way there, but
 out-of-tree builds required a second Makefile.am rule, which was
 missing @ARM2GNU_PARAMS@.
Also, the arm2gnu.pl was terminating argument processing on any
 argument beginning with --, rather than an argument that was just
 -- by itself (as is the normal convention in GNU programs).
That meant it never saw the --apple flag even when it was passed.

Thanks to Jonathan Lennox for the report and for testing.