[llvm-objdump] Implement -Mreg-names-raw/-std options.

The --disassembler-options, or -M, are used to customize
the disassembler and affect its output.

The two implemented options allow selecting register names on ARM:
* With -Mreg-names-raw, the disassembler uses rNN for all registers.
* With -Mreg-names-std it prints sp, lr and pc for r13, r14 and r15,
  which is the default behavior of llvm-objdump.

Differential Revision: https://reviews.llvm.org/D57680

llvm-svn: 354870