Add FixedLenDecoderEmitter, the skeleton of a new disassembler emitter for...

Add FixedLenDecoderEmitter, the skeleton of a new disassembler emitter for fixed-length instruction encodings.
A major part of its (eventual) goal is to support a much cleaner separation between disassembly callbacks
provided by the target and the disassembler emitter itself, i.e. not requiring hardcoding of knowledge in tblgen
like the existing disassembly emitters do.

The hope is that some day this will allow us to replace the existing non-Thumb ARM disassembler and remove
some of the hacks the old one introduced to tblgen.

llvm-svn: 125966