toolchain: fix detection of SSP support

toolchain: fix detection of SSP support

GCC issues a warning message if -fstack-protector is passed but SSP is
not available, so in order to force the compilation to fail we must also
pass -Werror.

All external toolchains were verified. The only one whose configuration
incorrectly selected BR2_TOOLCHAIN_HAS_SSP was CodeSourcery NIOSII.

Fixes:
  http://autobuild.buildroot.net/results/ce8fe8ac9cf0db01ae15d476ea714ff176965cfb
  http://autobuild.buildroot.net/results/09ce8f05e28c0219f499ce55130e896cae0c8b45

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>