Allow libffi snapshots

This is rather annoying. I'd prefer to have a stable release to
use. However libffi-3.2.1 has been released November 12, 2014, and
libffi-4 is TBD. See also https://github.com/libffi/libffi/issues/296

The core reason for this change is that llvm changed the supported
assembly to unified syntax, which libffi-3.2.1 does not use, and hence
fails to compile for arm with llvm. For refence, see the following
issue: https://github.com/libffi/libffi/issues/191.

This diff contains a script to generate a tarball for the
`libffi-tarballs` repository from the libffi GitHub repository; as well
as the necessary changes to the build system.

Updates libffi-tarballs submodule.

Reviewers: austin, bgamari, hvr

Subscribers: hvr, erikd, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3574

libffi-tarballs @ 81fbf2e7

-Subproject commit ec37a68838566cb830c33cb30bfade003a306cff
+Subproject commit 81fbf2e78a082c6ad464d4025b44402b3db8acdf

libffi/ghc.mk

@@ -123,7 +123,7 @@ $(libffi_STATIC_LIB): $(libffi_STAMP_INSTALL)
 	@test -f $@ || { echo "$< exists, but $@ does not."; echo "Suggest removing $<."; exit 1; }
 
 $(libffi_HEADERS): $(libffi_STAMP_INSTALL) | $$(dir $$@)/.
-	cp -f libffi/build/inst/lib/libffi-*/include/$(notdir $@) $@
+	cp -f libffi/build/inst/include/$(notdir $@) $@
 
 $(eval $(call clean-target,libffi,, \
     libffi/build $(wildcard libffi/stamp.ffi.*) libffi/dist-install))