Allow miniruby to load shared GC

Since dln.c is replaced with dmydln.c for miniruby, we cannot load shared
GC for miniruby. This means that many tests do not have the shared GC
loaded and many tests fail because of the warning that emits in miniruby.

This commit changes shared GC to directly use dlopen for loading the
shared GC.

gc.c

@@ -667,8 +667,10 @@ rb_gc_guarded_ptr_val(volatile VALUE *ptr, VALUE val)
 }
 #endif
 
-#if USE_SHARED_GC
-# include "dln.h"
+#if USE_SHARED_GC && !defined(HAVE_DLOPEN)
+# error "Shared GC requires dlopen"
+#elif USE_SHARED_GC
+#include <dlfcn.h>
 
 typedef struct gc_function_map {
     // Bootup
@@ -760,7 +762,7 @@ ruby_external_gc_init(void)
 
     char *gc_so_path = NULL;
     void *handle = NULL;
-    if (gc_so_file && dln_supported_p()) {
+    if (gc_so_file) {
         /* Check to make sure that gc_so_file matches /[\w-_.]+/ so that it does
          * not load a shared object outside of the directory. */
         for (size_t i = 0; i < strlen(gc_so_file); i++) {
@@ -781,17 +783,16 @@ ruby_external_gc_init(void)
         strcpy(gc_so_path + strlen(SHARED_GC_DIR), gc_so_file);
         gc_so_path[strlen(SHARED_GC_DIR) + strlen(gc_so_file)] = '\0';
 
-        char error[1024];
-        handle = dln_open(gc_so_path, error, sizeof(error));
+        handle = dlopen(gc_so_path, RTLD_LAZY | RTLD_GLOBAL);
         if (!handle) {
-            fprintf(stderr, "%s", error);
+            fprintf(stderr, "%s", dlerror());
             rb_bug("ruby_external_gc_init: Shared library %s cannot be opened", gc_so_path);
         }
     }
 
 # define load_external_gc_func(name) do { \
     if (handle) { \
-        rb_gc_functions.name = dln_symbol(handle, "rb_gc_impl_" #name); \
+        rb_gc_functions.name = dlsym(handle, "rb_gc_impl_" #name); \
         if (!rb_gc_functions.name) { \
             rb_bug("ruby_external_gc_init: " #name " func not exported by library %s", gc_so_path); \
         } \
@@ -4649,12 +4650,6 @@ rb_obj_info_dump_loc(VALUE obj, const char *file, int line, const char *func)
 void
 Init_GC(void)
 {
-#if USE_SHARED_GC
-    if (getenv(RUBY_GC_LIBRARY) != NULL && !dln_supported_p()) {
-        rb_warn(RUBY_GC_LIBRARY " is ignored because this executable file can't load extension libraries");
-    }
-#endif
-
 #undef rb_intern
     malloc_offset = gc_compute_malloc_offset();