[mips] Fold MipsTargetInfoBase subclasses into MipsTargetInfoBase and rename to MipsTargetInfo. NFC

Summary:
This unifies mips/mipsel and mips64/mips64el into a single class so that we can
later support O32 on mips64/mips64el and N32/N64 on mips/mipsel (when an
appropriate CPU selected).

Reviewers: atanasyan

Subscribers: atanasyan, jfb, cfe-commits, dschuff

Differential Revision: http://reviews.llvm.org/D20678

llvm-svn: 270984