kbuild: Use -nostdinc in compile tests

kbuild: Use -nostdinc in compile tests

gcc 4.8 and later include <stdc-predef.h> by default.  In some
versions of eglibc that includes <bits/predefs.h>, but that may be
missing when building with a biarch compiler.  Also <stdc-predef.h>
itself could be missing as we are only trying to build a kernel, not
userland.

The -nostdinc option disables this, though it isn't explicitly
documented.  This option is already used when actually building
the kernel, but not by cc-option and other tests.  This can result
in silently miscompiling the kernel.

References: https://bugs.debian.org/717557
References: https://bugs.debian.org/726861
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name kbuild-use-nostdinc-in-compile-tests.patch