Add disassembly to machine code

- FuncMCView wraps the memory block in MemoryObject interface.
- The slightly tricky thing is to get the in-memory size of the final
asm. This is not exposed by the ExecutionEngine, but conveniently it is
stored already in jl_jit_events FuncInfo structure.
- Uses MCDisassembler, implementation directly derived from code in
llvm-mc.