Rework how iOS does linking (#8127)

iOS has some particular constraints about how applications can be built:

 * We must generate a static library (.a) since XCode does the final
   link.
 * We need to carefully give the right set of arguments to libtool in
   the case we're generating an archive.
 * Dynamic linking isn't supported.
 * It can only be done on OS X.

This patch cleans up all of the above. We add a new flag `-staticlib`
(only supported on Darwin) that allows us to produce archive files using
libtool, and a -pgmlibtool flag to control which 'libtool' executable to
use.

This fixes #8127. I believe this is the last piece missing from the iOS
cross compiler.

Authored-by: Luke Iannini <lukexi@me.com>
Authored-by: Maxwell Swadling <maxwellswadling@gmail.com>
Authored-by: Stephen Blackheath <...@blacksapphire.com>
Signed-off-by: Austin Seipp <aseipp@pobox.com>