Makefile: Prepare for clang EFI builds

clang complains about GNU extensions such as variable sized types not
being at the end of structs unless -Wno-gnu is used. We may
eventually want -Wno-gnu, but for now let's just handle the warnings
as they come. Add -Wno-gnu-variable-sized-type-not-at-end to avoid
the warning issued for the initrd_dev_path struct. (Eliminating the
warning is preferred to reworking the struct, because the
implementation is imported verbatim from Linux.)

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>