samples/xtensa-asm2: Unit test for new Xtensa assembly primitives

This sample (which should eventually become a proper test) suite
builds from simple applications of the new primitives to a full
context switch test and interrupt handling suite (based on the
CPU-internal CCOMPARE2 timer).

It's been extraordinarily useful finding regressing as the asm2 code
gets modified and should probably stick around as long as possible.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>