gdb: error if /r and /b are used with disassemble command

The disassembler gained a new /b flag in this commit:

  commit d4ce49b7ac077a9882d6a5e689e260300045ca88
  Date:   Tue Jun 21 20:23:35 2022 +0100

      gdb: disassembler opcode display formatting

The /b and /r flags result in the instruction opcodes displayed in
different formats, so it's not possible to have both at the same
time.  Currently the /b flag overrides the /r flag.

We have a similar situation with the /m and /s flags, but here, if the
user tries to use both flags then they will get an error.

I think the error is clearer, so in this commit I propose that we add
an error if /r and /b are both used.

Obviously this change breaks backwards compatibility.  I don't have a
compelling argument for why we should make the change beyond my
feeling that it was a mistake not to add this error from the start,
and that the new behaviour is better.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>