Add check-asm for fixed-point xcorr_kernel().

Compare the output of xcorr_kernel() against the results of
 xcorr_kernel_c() when configured with --enable-check-asm.
Currently this is only checked in fixed point, as a float check
 requires more sophisticated error analysis and may need to be
 customized for each vector implementation.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>