toolchain/toolchain-external/toolchain-external-custom: add option to indicate SSP_ST...

toolchain/toolchain-external/toolchain-external-custom: add option to indicate SSP_STRONG support

This commit adds a user-visible option
BR2_TOOLCHAIN_EXTERNAL_HAS_SSP_STRONG, which will allow the user to
indicate if the custom external toolchain does or does not have
SSP_STRONG support. Depending on this, the user will be able to use
(or not) the BR2_SSP_STRONG option.

Checking if what the user said is true or not about this is already
done in toolchain/toolchain-external/pkg-toolchain-external.mk:

        $$(Q)$$(call check_toolchain_ssp,$$(TOOLCHAIN_EXTERNAL_CC),$(BR2_SSP_OPTION))

If the user selects BR2_SSP_STRONG, this will check if
-fstack-protector-strong is really supported.

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