scs: Move DEFINE_SCS macro into core code

Defining static shadow call stacks is not architecture-specific, so move
the DEFINE_SCS() macro into the core header file.

Tested-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>