Allow bridge callbacks to be set after GC init

Allow bridge callbacks to be set after GC init

This is cleanup to the previous emergency fix for bug 42469. The goal of that fix was to defer bridge processor initialization until after sgen init, but it introduced a requirement mono_gc_register_bridge_callback be called before sgen init. This breaks Xamarin Designer, which sets up callbacks late.

This patch introduces an sgen_init_bridge () which is called both at startup and after callbacks are set, and performs the initialization once all pieces are in place. Additional concurrency and style issues are also addressed.