Config.in: security hardening: disable FORTIFY_SOURCE for gcc < 6

Config.in: security hardening: disable FORTIFY_SOURCE for gcc < 6

As reported in the bug report [1], gcc < 6 doesn't build when
FORTIFY_SOURCE is set to 1 or 2. The issue is related to the
upstream bug report [2] but the patch fixing the issue for gcc 6
has not been backported to earlier gcc versions.

Add a dependency on gcc at least version 6 to BR2_FORTIFY_SOURCE_1
and BR2_FORTIFY_SOURCE_2.

[1] https://bugs.busybox.net/show_bug.cgi?id=11476
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61164
[3] https://github.com/gcc-mirror/gcc/commit/55f12fce4ccf77513644a247f9c401a5b1fa2402

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
[Peter: only limit for internal toolchain as suggested by Matthew]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>