Update libffi.podspec

no_compact_unwind is no longer support since ld64-134.9:
http://opensource.apple.com/source/ld64/ld64-134.9/src/ld/Options.cpp

It should be removed from the Podspec as it causes builds to fail.
Merge pull request #2531 from alfonzzz/patch-1

Update libffi.podspec