Debugger: Fix disassembler to handle newer udis86 API.

In newer versions of udis86, the ud_t.mnemonic field is deprecated,
and one must now call ud_insn_mnemonic() to retrieve that information.
This was causing Debugger to not correctly identify subroutine calls
and jump instructions, which broke the stepping code in various fun
ways.
Debugger: Adapt x86_64 disassembler to newer udis API.