sim: make compile on FreeBSD prior to 11

sim: make compile on FreeBSD prior to 11

FreeBSD before the early 11 development is expecting a char *
for stack.ss_sp rather than the standards compliant void *.
Catch that case and allow gem5 to compile on old FreeBSD versions.

Change-Id: Ic0ae560b52bfe5b3905ae87df791d349e053ec97
Reviewed-on: https://gem5-review.googlesource.com/4660
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>