stackleak: allow to specify arch specific stackleak poison function

Factor out the code that fills the stack with the stackleak poison value
in order to allow architectures to provide a faster implementation.

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230405130841.1350565-2-hca@linux.ibm.com


Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>