Dear maintainers,

The protobuf package was built on loong64.
But I found that there is a lack of loongarch64 support in
src/google/protobuf.

If the support for loong64 is missing in protobuf, packages that
build-depend on protobuf will fail to build, for example,
Compiling the justbuild(Need to add loongarch64 support first) failed
for loong64.
The build error log is as follows,
```
......
In file included from work/google/api/http.pb.cc:4:
In file included from work/google/api/http.pb.h:23:
In file included from /usr/include/google/protobuf/io/coded_stream.h:131:
In file included from /usr/include/google/protobuf/stubs/common.h:47:
/usr/include/google/protobuf/stubs/platform_macros.h:98:1: error:
expected unqualified-id
    98 | GOOGLE_PROTOBUF_PLATFORM_ERROR
       | ^
......
```

Please consider the patch I attached for protobuf.
Based on attached patch, I have built protobuf successfully on locally.
Based on local libprotobuf-dev, I have built justbuild(Need to add
loongarch64 support first. I have submitted patch to justbuild source
pacakge) successfully on locally.
```
......
    dh_builddeb
dpkg-deb: building package 'justbuild' in
'../justbuild_1.5.1-1+loong64_loong64.deb'.
dpkg-deb: building package 'justbuild-dbgsym' in
'../justbuild-dbgsym_1.5.1-1+loong64_loong64.deb'.
  dpkg-genbuildinfo --build=binary
-O../justbuild_1.5.1-1+loong64_loong64.buildinfo
  dpkg-genchanges --build=binary
-O../justbuild_1.5.1-1+loong64_loong64.changes
```

Please add loongarch64 support in protobuf.
Your opinions are welcome.

Best regards,
Dandan Zhang