[mips] Add backend support for Mips32r[35] and Mips64r[35].

Summary:
These ISA's didn't add any instructions so they are almost identical to
Mips32r2 and Mips64r2. Even the ELF e_flags are the same, However the ISA
revision in .MIPS.abiflags is 3 or 5 respectively instead of 2.

Reviewers: vmedic

Reviewed By: vmedic

Subscribers: tomatabacu, llvm-commits, atanasyan

Differential Revision: http://reviews.llvm.org/D7381

llvm-svn: 229695