toolchain: add hidden BR2_TOOLCHAIN_HAS_SSP_STRONG boolean

This will allow toolchain to indicate if they support
-fstack-protector-strong or not.

Whenever the gcc version is >= 4.9, we always have SSP_STRONG support
if we have SSP support. However, some toolchains older than gcc 4.9
might have backported SSP_STRONG support, which is why we cannot rely
just on BR2_TOOLCHAIN_GCC_AT_LEAST_4_9.

Having this "default" value allows to avoid adding a "select
BR2_TOOLCHAIN_HAS_SSP_STRONG" in the internal toolchain logic plus in
almost external toolchains. But it allows custom external toolchains
that are pre-4.9 to potentially declare that they support strong SSP.

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