Fix libtool warning when compiling with bazel (#447)

If you build CgRPC with bazel, as done here,
https://github.com/bazelbuild/rules_swift/blob/master/third_party/com_github_grpc_grpc_swift/BUILD.overlay,
and you end up creating a library with libtool that contains it, you get
this warning for multiple files:

```
libtool: warning same member name (byte_buffer.o) in output file used for input files: .../libCgRPC.a(byte_buffer.o) and: .../libCgRPC.a(byte_buffer.o) due to use of basename, truncation and blank padding
```

This is because bazel translates both `server.c` and `server.cc` to
`server.o`. This fixes these warnings by appending `_shim` to the shim
files.