cmake: emu: qemu: start QEMU gdbserver only if relevant Kconfig is set

This removes the enforced `-s` option and only passes in a `-gdb` only if
CONFIG_QEMU_GDBSERVER_LISTEN_DEV is set. When unset, it allows users to
utilize QEMU_EXTRA_FLAGS from environment preventing a clash that occurs
if `-s` and `-gdb` are both passed to QEMU invocation.

Signed-off-by: Alp Sayin <alpsayin@gmail.com>