[mips] NaCl should now use the custom MipsELFStreamer (recently added) in spite
of MCELFStreamer.

This is so that changes to MipsELFStreamer will automatically propagate through
its subclasses.

No functional changes (MipsELFStreamer has the same functionality of MCELFStreamer
at the moment).

Differential Revision: http://llvm-reviews.chandlerc.com/D3130