xtensa: always install slow handler for unaligned access exception

Currently slow handler for unaligned access exception is not installed
when CPU has hardware support for unaligned access. However some opcodes
(e.g. l32ai, s32ri, s32c1i) would still raise unaligned access exception
even on such CPUs. In that case instead of SIGBUS and a diagnostic entry
in the kernel log the faulting process would receive SIGILL.
Always install slow handler for unaligned access exception to fix that.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>