Config.in: ensure BR2_SSP_STRONG can only be selected if supported

This commit ensures that BR2_SSP_STRONG cannot be chosen if the
toolchain doesn't support strong SSP.

Fixes:

  http://autobuild.buildroot.net/results/cba93a681d10692c4e4c5584e4c962bd18a608d4/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>