llvm-mc-fuzzer: A fuzzing tool for the MC layer.

Summary:
Only the disassembler is supported in this patch but it has already found a few
issues in the Mips disassembler (mostly invalid instructions being successfully
disassembled).

Reviewers: kcc

Subscribers: russell.gallop, silvas, kcc, llvm-commits

Differential Revision: http://reviews.llvm.org/D12723

llvm-svn: 247786