arch: xtensa: Fix arch_is_in_isr() race condition

Fixes a flaw in the the xtensa implementation of arch_is_in_isr()
that could manifest on SMP systems. If the reading of the current
CPU's nested interrupt count is not fully atomic on an SMP system,
then an ill-timed context switch could occur leaving the caller
reading the nested interrupt of a different CPU.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>