s390x: pgm interrupt handler and a way to test them

The program interrupt handler will detect unexpected program interrupts and
allow to expect + verify program interrupts for testing purposes.

We need "-fno-delete-null-pointer-checks", otherwise trying to access the
lowcore at address 0 makes GCC generate very weird code.

Add two tests to test for simple operation and addressing exceptions.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170531123925.4547-3-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>