gcc: remove unsafe patch check (poison system dirs) patch

Now that the calls to gcc always pass through the toolchain wrapper, it
is no longer necessary to patch gcc to support poisoning.

This does have the disadvantage that there is no unsafe path check for
libc, libgcc and libstdc++ (all of these are built before the wrapper
exists). But we can assume that the toolchain components themselves
should be pretty safe.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>