Fix stack-protector for powerpc

On powerpc, stack protector expects a function called
__stack_chk_fail_local() instead of __stack_chk_fail()

And some versions of GCC for powerpc default to TLS canary
instead of global canary, so always force GCC to use global
canary with -mstack-protector-guard=global

Cc: Joel Peshkin <joel.peshkin@broadcom.com>
Fixes: 4e9bce12 ("Add support for stack-protector")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>