[gn build] Use `=` for of -fdebug-compilation-dir

-f flags usually use the `=` form. -fdebug-compilation-dir= has been
around for a few months now (since 0c2bb6b4, both LLVM 12.0
and 13.0 have it), so using it shouldn't be a big problem -- especially
since use_relative_paths_in_debug_info is opt-in anyways.