configure: endianness check for cross compile

 - Add a cross compile check: assume yes if we can't run a test binary
 - If cross compiling, revert to a compile-time endianess check. This tries
   a few possible ways to detect big endian, but otherwise assumes little. We
   rely on the run-time check to save us if the build-time check is wrong.

Signed-off-by: Aaron Carroll <aaronc@cse.unsw.edu.au>
Signed-off-by: Jens Axboe <axboe@kernel.dk>