Summary:
Remove -Wl,--no-as-needed flag when making shared_lib in OSX and IOS as
those environment doe not have compile option --no-as-needed

  ld: unknown option: --no-as-needed
  clang: error: linker command failed with exit code 1 (use -v to see invocation)

Test Plan: make shared_lib

Reviewers: meyering, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D40353