common/ceph_context: Fix std::atomic<std::shared_ptr> compatibility

common/ceph_context: Fix std::atomic<std::shared_ptr> compatibility

Previously, we relied on the __GNUC__ macro to check for std::atomic<std::shared_ptr>
support, which was inaccurate. This approach failed with Clang builds using libstdc++,
even when the feature was implemented. The warning looks like:

```
/usr/lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/bits/shared_ptr_atomic.h:131:5: note: 'atomic_load_explicit<std::vector<entity_addrvec_t>>' has been explicitly marked deprecated here
  131 |     _GLIBCXX20_DEPRECATED_SUGGEST("std::atomic<std::shared_ptr<T>>")
      |     ^
```

This change uses a standard-compliant feature test macro (__cpp_lib_atomic_shared_ptr)
to correctly detect support for std::atomic<std::shared_ptr>. This resolves compilation
issues and improves portability across different compilers and standard library
implementations.

Refs 5b0d849730ce20d68ffafcb612c5f6fc8b87dd9a

Signed-off-by: Kefu Chai <tchaikov@gmail.com>