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.