sim-se: Correctly calculate next PC in clone

sim-se: Correctly calculate next PC in clone

The clone syscall doesn't propagate all state in the PCState object
when calculating the return PC of a newly created process. Instead of
creating a new PCState object from the next PC address, copy the old
PC and advance it.

Change-Id: Ice53831920bcb5d198865169ed2cca8d06e37cfe
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/15417
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>