Disable -Wzero-length-bounds for the kernel for GCC 12.

[jhb]

Summary

The mlx5 driver and some other OFED bits use a somewhat dubious
pattern of:

struct foo {
    uint64_t arg[0];
    /* Real members of a struct */
};

The code then treats 'arg' as if it were really a kind of union
to such that foo.arg[N] functions similarly to (uint64_t *)foo[N].

No real bugs were found by this warning though, so just turn it off
globally.

[kib]

This is undefined behavior for sure.

[mjg]

is there a problem disabling it just for the known problematic code?

[jhb]

In D37630#855698, @mjg wrote:

is there a problem disabling it just for the known problematic code?

There was a lot to disable, and I found no real bugs. That is, for amd64's GENERIC today it only yielded false positives.