[gdb] Fix warning in print_one_insn::ez80-adl

When running selftest print_one_insn::ez80-adl we run into this warning:
...
Running selftest print_one_insn::ez80-adl.
warning: Unable to determine inferior's software breakpoint type: couldn't
  find `_break_handler' function in inferior. Will be used default software \
  breakpoint instruction RST 0x08.
...

Fix this by explicitly handling bfd_arch_z80 in print_one_insn_test.

Tested on x86_64-linux.