toolchain: -fstack-protector-strong can be back-ported

toolchain: -fstack-protector-strong can be back-ported

Currently, use of -fstack-protector-strong is only available for gcc
starting with 4.9, on the assumption that it appeared with that version.

Although this is true, it happens that quite a few vendors will have
back-ported -fstack-protector-strong to older gcc versions (at least 4.8
seen in the wild).

Remove the guard against gcc>=4.9, and expand the help text.

Note: we could have changed the guard to something like:
    depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_9 || BR2_TOOLCHAIN_EXTERNAL_CUSTOM

However, the latest gcc we support in the internal toolchain now *is*
gcc-4.9, and similarly all external toolchains except Sourcery ARM are
4.9 or higher. So except for the Sourcery toolchain, the condition would
have always been true. For that one toolchain, we can allow it to hit
the SSP check, and just drop the condition entirely.

Signed-off-by: "Yann E. MORIN" <yann.morin@orange.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>