[xla:ffi] Add macro to export XLA:FFI handlers as C function symbols
[xla:ffi] Add macro to export XLA:FFI handlers as C function symbols

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/13241 from openxla:fail_on_result_conflict 99192a7119725e1b80153f8b29a229acb191c4f7
