If the compiler happens to pick `ret = r0`, then this will assemble to
`ag r0, 0` which is obviously not what we want. Using `a` instead of `r` will
ensure that we get an appropriate address register, i.e. `r1` through `r15`.

Re-enable pie_linux for s390x-linux which was disabled in
ed7ff0b693037078f451a7c6c1124611060f4892.