sys: Disable -Walloc-size-larger-than for GCC 9+

By default this warns about sizes larger than PTRDIFF_MAX passed to
malloc (rather than SIZE_MAX). This doesn't trigger
deterministically, but it does trigger for kmalloc() of struct_size()
in iwlwifi's iwl_configure_rxq even when struct_size() is changed to
use PTRDIFF_MAX. NB: struct_size() in Linux caps the size at
SIZE_MAX, not PTRDIFF_MAX via size_mul().

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D42587