Link RCT-Folly against libc++abi

Summary:
Folly now depends on libc++abi. This solves linker error for RCT-Folly.podspec like this:

```
Undefined symbols for architecture arm64:
  "___cxa_increment_exception_refcount", referenced from:
      folly::exception_ptr_get_type(std::exception_ptr const&) in libRCT-Folly.a(Exception.o)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

See https://github.com/react-native-community/releases/issues/251

Note: RNTester was not affected by this bug for some reason, so the only way to verify is via the new app generated via `npx react-native init`.

Changelog: [Fixed][iOS] Unbreak Folly linker error

Reviewed By: lunaleaps

Differential Revision: D30950944

fbshipit-source-id: 3eb146e23faa308a02363761d08849d6801e21ca