sim-se: Fix mremap for downward growing mmap regions

sim-se: Fix mremap for downward growing mmap regions

mremapFunc(...) did not respect Process::mmapGrowsDown(). This resulted
in an attempt to remap into an already allocated region and a panic in
FuncPageTable::allocate(...). This behavior can be observed in
435.gromacs.

Change-Id: Ib3ad33816126c76506d69679bdcefa7a98ef69f9
Reviewed-on: https://gem5-review.googlesource.com/4700
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>