std.pie: fix register constraint in getDynamicSymbol() for s390x (#25327)

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
ed7ff0b693.

lib/std/pie.zig

@@ -177,7 +177,7 @@ inline fn getDynamicSymbol() [*]const elf.Dyn {
                 \\ jg 2f
                 \\ 1: .quad _DYNAMIC - .
                 \\ 2:
-                : [ret] "=r" (-> [*]const elf.Dyn),
+                : [ret] "=a" (-> [*]const elf.Dyn),
             ),
             // The compiler does not necessarily have any obligation to load the `l7` register (pointing
             // to the GOT), so do it ourselves just in case.