bpftool: Refactor disassembler for JIT-ed programs

Refactor disasm_print_insn() to extract the code specific to libbfd and
move it to dedicated functions. There is no functional change. This is
in preparation for supporting an alternative library for disassembling
the instructions.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Tested-by: Niklas Söderlund <niklas.soderlund@corigine.com>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20221025150329.97371-6-quentin@isovalent.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>