GC: Only force alloc slowpath for NEWOBJ hook

Previously, configuring any GC event hook would cause all allocations to
go through the newobj slowpath. We should only need to do that when the
newobj specifically is subscribed to.

This renames flags.has_hook to flags.has_newobj_hook, to make this new
usage clear. newobj_of0 was the only place which previously checked this
flag.

gc.c

@@ -747,7 +747,7 @@ typedef struct rb_objspace {
         unsigned int during_compacting : 1;
         unsigned int during_reference_updating : 1;
         unsigned int gc_stressful: 1;
-        unsigned int has_hook: 1;
+        unsigned int has_newobj_hook: 1;
         unsigned int during_minor_gc : 1;
         unsigned int during_incremental_marking : 1;
         unsigned int measure_gc : 1;
@@ -2466,7 +2466,7 @@ rb_objspace_set_event_hook(const rb_event_flag_t event)
 {
     rb_objspace_t *objspace = &rb_objspace;
     objspace->hook_events = event & RUBY_INTERNAL_EVENT_OBJSPACE_MASK;
-    objspace->flags.has_hook = (objspace->hook_events != 0);
+    objspace->flags.has_newobj_hook = !!(objspace->hook_events & RUBY_INTERNAL_EVENT_NEWOBJ);
 }
 
 static void
@@ -2476,7 +2476,7 @@ gc_event_hook_body(rb_execution_context_t *ec, rb_objspace_t *objspace, const rb
     EXEC_EVENT_HOOK(ec, event, ec->cfp->self, 0, 0, 0, data);
 }
 
-#define gc_event_hook_available_p(objspace) ((objspace)->flags.has_hook)
+#define gc_event_newobj_hook_needed_p(objspace) ((objspace)->flags.has_newobj_hook)
 #define gc_event_hook_needed_p(objspace, event) ((objspace)->hook_events & (event))
 
 #define gc_event_hook_prep(objspace, event, data, prep) do { \
@@ -2836,7 +2836,7 @@ newobj_of0(VALUE klass, VALUE flags, int wb_protected, rb_ractor_t *cr, size_t a
 
     if (!UNLIKELY(during_gc ||
                   ruby_gc_stressful ||
-                  gc_event_hook_available_p(objspace)) &&
+                  gc_event_newobj_hook_needed_p(objspace)) &&
             wb_protected) {
         obj = newobj_alloc(objspace, cr, size_pool_idx, false);
         newobj_init(klass, flags, wb_protected, objspace, obj);