lib: s390x: better smp interrupt checks

Use per-CPU flags and callbacks for Program and Extern interrupts,
instead of global variables.

This allows for more accurate error handling; a CPU waiting for an
interrupt will not have it "stolen" by a different CPU that was not
supposed to wait for one, and now two CPUs can wait for interrupts at
the same time.

This will significantly improve error reporting and debugging when
things go wrong.

Both program interrupts and external interrupts are now CPU-bound, even
though some external interrupts are floating (notably, the SCLP
interrupt). In those cases, the testcases should mask interrupts and/or
expect them appropriately according to need.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>