qemu, qemuv8: update help text and enable GDB for qemu_v8

- tee-supplicant is started automatically since commit 1333db4d
("rootfs: start tee-supplicant on boot")
- Update qemu_v8 arguments so that GDB can be used like for qemu

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>