s390x/selftest: Fix constraint of inline assembly

Clang on s390x compains:

/home/thuth/devel/kvm-unit-tests/s390x/selftest.c:39:15: error:
 %r0 used in an address
        asm volatile("  stg %0,0(%0)\n" : : "r"(-1L));
                     ^
<inline asm>:1:13: note: instantiated into assembly here
                stg %r0,0(%r0)
                          ^

Right it is. We should not use address register 0 for STG.
Thus let's use the "a" constraint to avoid register 0 here.

Message-Id: <20200924111746.131633-1-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>