Make API visibility hidden by default.

[ysiraichi]

This PR fixes #9691 by making PyTorch/XLA library visibility property hidden by default.
This is to make sure we don't share data unexpectedly with other libraries.

[ysiraichi]

Here's an intuition of why I think this fixes a previously undesirable PyTorch/XLA behavior:

• google::protobuf::internal::ShutdownData is a protobuf C++ class
• ShutdownData::get() is a singleton function that retrieves an instance of that class
  • It's stored in its function-local variable data

# ====================================================
# Before this PR :
#   - `data` is marked as a 'unique global symbol' (dynamic linker guarantees one copy per process)
#   - `data` is shared with other libraries that also have this symbol
#
$ nm -C _XLAC.cpython-312-x86_64-linux-gnu.so | grep ShutdownData
0000000017351dd0 u guard variable for google::protobuf::internal::ShutdownData::get()::data
0000000017351dd8 u google::protobuf::internal::ShutdownData::get()::data

# ====================================================
# After this PR:
#   - `data` is a local symbol visible to this library only
#   - Other libraries can't get access to `data`
#
$ nm -C  _XLAC.cpython-312-x86_64-linux-gnu.so | grep ShutdownData
000000000d694c58 b guard variable for google::protobuf::internal::ShutdownData::get()::data
000000000d694c60 b google::protobuf::internal::ShutdownData::get()::data

[jeffhataws]

@ysiraichi @qihqi will you cherry-pick to r2.9 branch? Thanks.