Disable some clang warnings

Disable some clang warnings that can't be fixed in the code easily.

The gnu-variable-sized-type-not-at-end extension is used heavily to
allow adding members to a struct after a variably sized header.

constant-logical-operand is triggered by code in l2c_utils.c that looks
like the construct is correct in this use case.

Change-Id: I3ff8bf5b6deb7a5807dc200a50f33b2583abb9c0
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>