Xtensa port: Restore interrupted context the same way as premption resume.

This was a kind of optimization that save few cycles in the cost of code
duplication. However the path where we resume from interrupt withouch changing
the running thread is buggy and leads to many tests to fail. For now I'd prefer
to remove this optimization and have a working port. Later, once everything is
OK, I'll have time to optimize things.

Change-Id: I7af58f383848d157b9f3b3fbeceede3e83f9ce61
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>