[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215542 91177308-0d34-0410-b5e6-96231b3b80d8