[mips][mips16] MIPS16 is not a CPU/Architecture but is an ASE.

Summary:
The -mcpu=mips16 option caused the Integrated Assembler to crash because
it couldn't figure out the architecture revision number to write to the
.MIPS.abiflags section. This CPU definition has been removed because, like
microMIPS, MIPS16 is an ASE to a base architecture.

Reviewers: vkalintiris

Subscribers: rkotler, llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D13656

llvm-svn: 250407