Conditionally add -fno-stack-protector to CFLAGS

Conditionally add -fno-stack-protector to CFLAGS

When compile-testing on powerpc, I get errors like this:

net/nfs.c:422: undefined reference to `__stack_chk_fail_local'

This seems to be because -fstack-protector is on by default, so
let's explicitly disable it on all architectures that support the
option.

The Ubuntu toolchain is affected by this problem, and according to
Mike Frysinger, Gentoo has been running with SSP enabled for years.
More and more distros are turning SSP on by default, so this problem
is likely to get worse in the future.

Also, powerpc just happens to be one of the arches I do
compile-testing on. There may be other arches affected by this too.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>