[mips] Replace almost all Arch checks in MipsTargetInfo with ABI checks. NFC.

Summary:
setABI() is still tied to the Arch component of the Triple to preserve existing
behaviour.

Reviewers: atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D20961

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271875 91177308-0d34-0410-b5e6-96231b3b80d8