Allow external use of keep-alive API

The current implementation is a macro which means it cannot be used
directly in Alloy.

include/gc/gc.h

@@ -1518,6 +1518,13 @@ GC_API int GC_CALL GC_invoke_finalizers(void);
 # define GC_reachable_here(ptr) GC_noop1((GC_word)(ptr))
 #endif
 
+/* Explicitly tell the collector that an object is reachable    */
+/* at a particular program point.  This prevents the argument   */
+/* pointer from being optimized away, even it is otherwise no   */
+/* longer needed. Used to prevent finalizers from running when  */
+/* the associated object is still in use.                       */
+GC_API void GC_CALL GC_keep_alive(GC_word);
+
 /* Make the argument appear live to compiler.  Should be robust against */
 /* the whole program analysis.                                          */
 GC_API void GC_CALL GC_noop1(GC_word);

mark.c

@@ -54,6 +54,11 @@ GC_API void GC_CALL GC_noop1(GC_word x)
 # endif
 }
 
+GC_API void GC_keep_alive(GC_word x)
+{
+  GC_reachable_here(x);
+}
+
 /* Initialize GC_obj_kinds properly and standard free lists properly.   */
 /* This must be done statically since they may be accessed before       */
 /* GC_init is called.                                                   */