Adding a C API to the disassembler for use by such tools as Darwin's otool(1).

This is a work in progress as the interface for producing symbolic operands is
not done.  But a hacked prototype using information from the object file's
relocation entiries and replacing immediate operands with MCExpr's has been
shown to work with no changes to the instrucion printer.  These APIs will be
moved into a dynamic library at some point.

llvm-svn: 128308