s390: Add -m64 to KBUILD_CPPFLAGS

Some non-code files, like linkescripts, are preprocessed with the C
preprocessor and make use of regular kernel headers.
As -m64 is not passed to those preprocessor invocations this leads
to an inconsistency between __BITS_PER_LONG and the C type 'long'.
An upcoming consistency check will be tripped by this.

Make sure -m64 is also defined for those preprocessing steps.

As KBUILD_CPPFLAGS is inherited by both KBUILD_AFLAGS and KBUILD_CFLAGS,
drop -m64 from these variables.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260302-vdso-compat-checkflags-v2-3-78e55baa58ba@linutronix.de