gcc: make include poisoning fatal again in gcc/g++

gcc: make include poisoning fatal again in gcc/g++

We have a patch to allow us to 'poison' system include directories,
which are warnings by default but we make them fatal in cross builds.

However, in the 13.1 upgrade[1] the patch to make the warnings fatal was
dropped in the compiler invocation, so it only took effect for pure
preprocessor calls. This was not noticed at the time as the test case
was flawed, but this has now been fixed.

Add back the fatal poisoning, and restructure the patch slightly so it
is less invasive.

[1] oe-core bea46612fd9106cc5b46eb1d81623b6492563c13

[RP: Tweak to fix gcc/gcc-cross-canadian failure]
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>