ppc64/purgatory: Disabling GCC's stack protection

Some Linux distributions, like Suse, are turning on the GCC's stack
protection mechanism by default (-fstack-protector). When building the
purgatory with this option, this leads to link issues that are revealed at
runtime when the purgatory is loaded because symbols like __stack_chk_fail
are unresolved.

This patch forces this stack protection mechanism to be turned off when
building the purgatory on ppc64 BE and LE.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>