toolchain: improve SSP logic

toolchain: improve SSP logic

Don't enable SSP support on external toolchains just because they use
glibc or musl. Instead of that, make the external toolchains explictily
declare if they support SSP or not. And also add a check to detect SSP
support when using custom external toolchains.

For internal toolchains we always enable SSP support for glibc and musl.

Fixes:

  http://autobuild.buildroot.net/results/ac7c9b3ad2e52abfe6b79a80045e4218eeb87175/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[Thomas:
 - remove uClibc-specific SSP check, since there is now a generic
   check being done.
 - send potential compilation errors caused by the SSP check to
   oblivion, in order to avoid causing confusion for the user.
 - add autobuilder reference.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>