Non-zero exit when error in loading shared GC

Before this commit, when there is an error in loading the shared GC, an
error was outputted but it did not exit, causing it to segfault later on.

Author: peterzhu2118

gc.c

@@ -648,8 +648,6 @@ static rb_gc_function_map_t rb_gc_functions;
 
 # define RUBY_GC_LIBRARY "RUBY_GC_LIBRARY"
 
-# define fatal(...) do {fprintf(stderr, "" __VA_ARGS__); return;} while (0)
-
 static void
 ruby_external_gc_init(void)
 {
@@ -672,7 +670,8 @@ ruby_external_gc_init(void)
               case '.':
                 break;
               default:
-                fatal("Only alphanumeric, dash, underscore, and period is allowed in "RUBY_GC_LIBRARY"");
+                fprintf(stderr, "Only alphanumeric, dash, underscore, and period is allowed in "RUBY_GC_LIBRARY"\n");
+                exit(1);
             }
         }
 
@@ -682,7 +681,8 @@ ruby_external_gc_init(void)
 
         handle = dlopen(gc_so_path, RTLD_LAZY | RTLD_GLOBAL);
         if (!handle) {
-            fatal("ruby_external_gc_init: Shared library %s cannot be opened: %s", gc_so_path, dlerror());
+            fprintf(stderr, "ruby_external_gc_init: Shared library %s cannot be opened: %s\n", gc_so_path, dlerror());
+            exit(1);
         }
     }
 
@@ -692,7 +692,8 @@ ruby_external_gc_init(void)
     if (handle) { \
         gc_functions.name = dlsym(handle, "rb_gc_impl_" #name); \
         if (!gc_functions.name) { \
-            fatal("ruby_external_gc_init: " #name " func not exported by library %s", gc_so_path); \
+            fprintf(stderr, "ruby_external_gc_init: " #name " func not exported by library %s\n", gc_so_path); \
+            exit(1); \
         } \
     } \
     else { \