[mips] Refactor calls to setCanHaveModuleDir.

Summary:
Moved some calls to setCanHaveModuleDir to the MipsTargetStreamer base class and removed the resulting empty functions from the MipsTargetELFStreamer class.

Also fixed a missing call to setCanHaveModuleDir in MipsTargetELFStreamer::emitDirectiveSetMicroMips.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: tomatabacu

Differential Revision: http://reviews.llvm.org/D4781