lib/s390x: Fix the epsw inline assembly

According to the Principles of Operation, the epsw instruction
does not touch the second register if it is r0. With GCC we were
lucky so far that it never tried to use r0 here, but when compiling
the kvm-unit-tests with Clang, this indeed happens and leads to
very weird crashes. Thus let's make sure to never use r0 for the
second operand of the epsw instruction.

While we're at it, also change the constraint modifier from "+" to
"=" since these are only output parameters, not input.

Message-Id: <20210622135517.234801-4-thuth@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>