vppinfra: add CLIB_ASSUME() macro

vppinfra: add CLIB_ASSUME() macro

This macro privides a way to tell compiler that it is safe to assume
that specified expression is true so it can optimize code accordingly.

i.e.

    CLIB_ASSUME (n < 3);

    while (n)
    {
      /* ... */
    }

Will tell compiler that n is never going to be >= 3 so instead of
creating loop it will just unroll loop 2 times.

Type: improvement
Change-Id: I718a9b95ff7980d7ac68eb9a88357a4ab6eee74a
Signed-off-by: Damjan Marion <damarion@cisco.com>