Generate .debug symbols for libfdb_c

Currently, it appears that [libfdb_c.so is stripped](https://github.com/apple/foundationdb/blob/297d831192b797157851125a71af4deec9a4773c/bindings/c/CMakeLists.txt#L50), and that this stripping does not generate a .debug file with symbols, [unlike what is done for executables](https://github.com/apple/foundationdb/blob/07e531947765696c3d0e80703967dd5da420fb28/cmake/FlowCommands.cmake#L136).

This is problematic for consumers of libfdb_c.so, because there's no way to symbolicate a stack trace, and the FDB project cannot offer a .debug file with symbols (like how is done for all the server components), because no such file exists.