runners: nsim: allow to use runner if gdb is missing

As of today we check for gdb presence in 'do_create' method and
raise an exception in case of gdb is missing. That makes nsim
runner unusable without gdb even for the commands which don't use
it (like 'flash' command).

Fix that.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>