[libcxx] Fix check-cxx-abilist on OS X

Summary:
Recent commits broke the check-cxx-abilist by changing the default OS X to use `-rexport_library` instead of `-reexport_symbol_list`. Apparently `-reexport_library` doesn't export the symbols into `libc++.dylib`s symbol table, whereas `-reexport_symbol_list` does.

This means the change removed ~500 symbols from the symbol table. I've been told this change is non ABI breaking, but it does make it harder to maintain the ABI lists, and hence the ABI.

This patch fixes the issue by switching back to `-reexport_symbol_list`. It still avoid the issues fixed in r299052 by putting the new/delete symbols in a different symbol list file, which is only exported when LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS in OFF.


Reviewers: mehdi_amini, smeenai, dexonsmith

Reviewed By: smeenai

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D31644

llvm-svn: 300390