stage2 ARM: amend implementation of various AIR instructions

- unwrap_errunion_err for registers
- unwrap_errunion_payload for registers
- ptr_slice_len_ptr for all MCValues
- ptr_slice_ptr_ptr for all MCValues

test/behavior/struct.zig

@@ -104,7 +104,6 @@ fn testMutation(foo: *StructFoo) void {
 
 test "struct byval assign" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
 
     var foo1: StructFoo = undefined;
     var foo2: StructFoo = undefined;