kexec/ppc64: disabling exception handling when building the purgatory

Some Linux distributions would like to turn on the GCC exception handling
by default. As this option introduces symbols in the built code that are
defined in a separate shared library, this is not a good idea to have such
an option activated when building the purgatory.

This patch forces the exception handling 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>