s390x: Move the value out of the casloop_val_reg with mov_preg (#5430)

The casloop_emit function in the s390x backend was using the fixed non-allocatable register %r0 directly with move instructions, which produced a panic in the regalloc2 checker (#5425). This PR changes the casloop_result function to use mov_preg instead of copy_reg to fetch the result, as it's not viewed by regalloc2 as a move.

Fixes #5425

cranelift/filetests/filetests/isa/s390x/issue-5425.clif

@@ -0,0 +1,16 @@
+test compile
+set regalloc_checker=1
+target s390x
+
+function %a() system_v {
+    fn0 = %callee_f64(i64) -> i32
+
+block0:
+    v1 = iconst.i64 0
+    v2 = call fn0(v1)  ; v1 = 0
+
+    v21 = iconst.i64 0
+    v22 = iconst.i32 2
+    v23 = atomic_rmw.i32 xchg v21, v22  ; v21 = 0, v22 = 2
+    trap user0
+}