Fix an endian problem in upb that caused protoc plugins to crash on big endian platforms (#25363)

The protobuf build started failing on big endian platforms (I'm using s390x) starting with v33.0. The cmake build would fail when running `protoc` to compile test files. The errors looked like:

```bash
gmake[2]: *** [CMakeFiles/upb-test.dir/build.make:85: google/protobuf/any.upb.h] Error 1
--upb_out: protoc-gen-upb: Plugin killed by signal 11.
```

I did a git bisect and the crash started with commit c36f728. The fix is to add a `_upb_Decoder_MungeInt32(val)` call on the closed enum value so it can be decoded properly later.

I think this might fix #24103 but I'm not able to test it.

Closes #25363

COPYBARA_INTEGRATE_REVIEW=#25363 from linux-on-ibm-z:upb-decode-endian 7143834
PiperOrigin-RevId: 865398323

Author: jonathan-albrecht-ibm

upb/wire/decode.c

@@ -938,6 +938,7 @@ const char* _upb_Decoder_DecodeWireValue(upb_Decoder* d, const char* ptr,
           *op = kUpb_DecodeOp_UnknownField;
           return ptr;
         }
+        _upb_Decoder_MungeInt32(val);
       } else {
         _upb_Decoder_Munge(field, val);
       }