toolchain: check the SSP option is known

toolchain: check the SSP option is known

Some toolchain vendors may have backported those options to older gcc
versions, and we have no way to know, so we have to check that the
user's selection is acceptable.

Extend the macro that currently checks for SSP in the toolchain, with
a new test that the actual SSP option is recognised and accepted.

Note that the SSP option is either totaly empty, or an already-quoted
string, so we can safely and easily assign it to a shell variable to
test and use it.

Note that we do not introduce BR2_TOOLCHAIN_HAS_SSP_STRONG, because:

  - our internal toolchain infra only supports gcc >= 4.9, so it has
    SSP strong;

  - of the external pre-built toolchains, only the codesourcery-arm
    one has a gcc-4.8 which lacks SSP strong, all the others have a
    gcc >= 4.9;

  - we'd still have to do the actual check for custom external
    toolchains anyway.

So, we're not adding BR2_TOOLCHAIN_HAS_SSP_STRONG just for a single
case.

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>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>