xtensa: add kconfig to allow non-preemptible interrupts

This adds a kconfig to enable making the interrupts
non-preemptible by other interrupts. Enabling this will set
the INTLEVEL to the max non-debug level before clearing
the EXCM bit.

Signed-off-by: Christopher J. Champagne <christopher.j.champagne@intel.com>
Signed-off-by: Daniel Leung <daniel.leung@intel.com>