Disable stack protector on all architectures

OpenBIOS currently fails to link with stack protector enabled, as it is
unlikely needed in boot loader anyway, just make sure it is disabled on
all targets.

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>