armv8r64-extras/refstack-tests: Move shebang to first line

Shebang needs to be in the first line of the file.

Issue-Id: SCM-4357
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Ic3b903d4c84edf0b01ad3496f297c97ac823f035