arch-arm, sim-se: Fix incorrect SP handling in clone

arch-arm, sim-se: Fix incorrect SP handling in clone

The clone syscall is currently broken on aarch64 since the aarch64
code uses an incorrect SP register. Fix this by storing the new stack
pointer in SP_EL0 instead of R13.

Change-Id: Ie17990b4f359608e3b53e5bf625eca53769a6653
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Cc: Giacomo Travaglini <giacomo.travaglini@arm.com>
Cc: Javier Setoain <javier.setoain@arm.com>
Cc: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15436
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>