[mips] Mark microMIPS64 as being unsupported.

There are no provided instruction definitions for this architecture.

Reviewers: smaksimovic, atanasyan, abeserminji

Differential Revision: https://reviews.llvm.org/D48320


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335057 91177308-0d34-0410-b5e6-96231b3b80d8