[builtins] Add __emutls_unregister_key function

This is called by Bionic on dlclose to delete the emutls pthread key.

The return value of pthread_key_delete is unchecked and behaviour of
multiple calls to the method is dependent on the implementation of
pthread_key_delete.

Differential Revision: https://reviews.llvm.org/D52251

llvm-svn: 342608