kexec-tools: disable the GCC's stack protection for purgatory code

kexec-tools: disable the GCC's stack protection for purgatory code

If the GCC's stack protection is enabled by default, the purgatory will
also be built with this option. But it makes no sense to enable this
for the purgatory code, and would cause error when we are trying to
relocate the purgatory codes because symbol like __stack_chk_fail is
unresolved. Instead of disabling this for some archs specifically,
disable it for all the archs.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>