xtensa: add support for cores without NMI

Some Xtensa cores do not support NMI, so XCHAL_HAVE_NMI=0 and
XCHAL_NMILEVEL won't be defined at all causing
arch/xtensa/include/xtensa-asm2-s.h to throw compilation error.

Fixes: #67855

Signed-off-by: Maciej Kusio <maciejkusio@meta.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>