xtensa: fix getting exccause during backtrace

We have frame pointer struct and BSA struct to extract
the exception cause (exccause). There is no need to
resort to custom assembly to do that. Besides, given
that the BSA is different between different Xtensa cores,
there is no guarantee it is at the same place as what
the assembly assumes. So just do that without assembly.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>