move the call to mono_gc_base_init into mono_debug_init

move the call to mono_gc_base_init into mono_debug_init

mono_debug_init is a public embedding interface and calls into
mono_debugger_initialize, which registers a GC root.  driver.c calls
mono_gc_base_init before calling mono_debug_init for this reason, but
that's not an option for embedding (as mono_gc_base_init is internal.)

since mono_gc_base_init is idempotent, just add a call to
mono_debug_init (and remove the driver.c call).  Document (in sgen-gc
and sgen-gc.h) the fact that although mono_gc_base_init is idempotent,
changing LOCK_INIT to something non-idempotent will break this.