toolchain: move Stack Protection Support option

The BR2_USE_SSP option is only used inside the uClibc build, so only
meaningful for Buildroot internal toolchains. Therefore, the option is
moved to the right location so that it isn't visible when working with
external toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>