Makefile: Globally enable VLA warning

Makefile: Globally enable VLA warning

Now that Variable Length Arrays (VLAs) have been entirely removed[1]
from the kernel, enable the VLA warning globally. The only exceptions
to this are the KASan an UBSan tests which are explicitly checking that
VLAs trigger their respective tests.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Cc: Masahiro Yamada <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: David Airlie <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>