Add pthread Option in Bazel C++ Toolchain Configuration

[nradakovic]

Description

In order to support multithreaded C++ applications, we need to ensure that the Bazel C++ toolchain includes support for the pthread library.

This involves updating the toolchain configuration to add the appropriate compiler and linker flags (-pthread) so that developers can build applications that rely on POSIX threads without needing to manually specify these options in their BUILD files.

Acceptance Criteria:

• The Bazel C++ toolchain includes -pthread in link actions.
• C++ targets using pthread build successfully without requiring manual flag injection.
• The change does not affect targets that do not require threading.
• The update is documented clearly as part of the toolchain configuration.

Additional Notes:

• This change is important for enabling portability and correctness when working with threaded applications.
• It should be compatible with existing modules and not introduce regressions in non-threaded code.