[orc-rt] Add C API for Errors, plus ORC_RT_C_ABI macro. (#178794)

This commit introduces a C interface for the ORC runtime's Error
handling system, enabling C clients and language bindings to work with
ORC errors.

The ORC_RT_C_ABI macro applies __attribute__((visibility("default")))
(on platforms that support it), ensuring C API symbols are exported when
building the ORC runtime as a shared library. In the future I expect
that this will be extended to support other platforms (e.g. dllexport on
Windows).

Author: lhames