Add some scaffolding for a new experimental asmprinter

implementation.  The idea is that we want asmprinting to
work by converting MachineInstrs into a new MCInst class,
then the per-instruction asmprinter works on MCInst.  MCInst
and the new asmprinters will not depend on most of the 
llvm code generators.  This allows building diassemblers
that don't link in the whole llvm code generator.  This is
step #1 of many.

llvm-svn: 73743