Codegen fix atomic_cas with samll types on riscv64 (#5919)

* fix issue5901

* add regression test file.

* fix regression targets.

* fix a comment.

* enable atomic-cas-little for riscv64

* specify little endian some s390x can pass test.

* fix register error

cranelift/filetests/filetests/runtests/atomic-cas-little.clif

@@ -1,6 +1,7 @@
 test interpret
 test run
 target s390x
+target riscv64 has_a 
 
 ; We can't test that these instructions are right regarding atomicity, but we can
 ; test if they perform their operation correctly

cranelift/filetests/filetests/runtests/issue5901.clif

@@ -0,0 +1,21 @@
+test interpret
+test run
+target aarch64
+target s390x
+target x86_64
+target riscv64
+
+function %a(i8) -> i8 sext system_v {
+    ss2 = explicit_slot 2
+
+block0(v0: i8):
+    v2 = iconst.i16 0x00ff
+    v3 = stack_addr.i64 ss2
+    store little v2, v3
+
+    v4 = stack_addr.i64 ss2+1
+    v5 = atomic_cas little v4, v0, v0
+    return v5
+}
+
+; run: %a(0) == 0