libeth: xdp, xsk: access adjacent u32s as u64 where applicable

On 64-bit systems, writing/reading one u64 is faster than two u32s even
when they're are adjacent in a struct. The compilers won't guarantee
they will combine those; I observed both successful and unsuccessful
attempts with both GCC and Clang, and it's not easy to say what it
depends on.
There's a few places in libeth_xdp winning up to several percent from
combined access (both performance and object code size, especially
when unrolling). Add __LIBETH_WORD_ACCESS and use it there on LE.
Drivers are free to optimize HW-specific callbacks under the same
definition.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>