libeth: xdp: add templates for building driver-side callbacks

Defining driver-specific functions to pass to libeth_xdp functions can
induce boilerplates and/or look a bit cryptic with all those layers of
indirection. On the other hand, this indirection is needed to allow
compilers to uninline big functions even when passed to __always_inline
helpers (too much inlining also hurts performance in some cases), plus
to reuse some XDP helpers in XSk code.
Add macros to quickly build them, with the detailed kdoc. They take
names of the actual callbacks for filling a Tx descriptor and other
purely HW-specific things and wrap them appropriately.

LIBETH_XDP_DEFINE_{BEGIN,END}() is needed for GCC 8+ unfortunately to
let the drivers control which functions will be static and which global
without hitting `-Wold-style-declaration`.

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