Fix: register GC callbacks inside GC.init (#15278)

Running the std specs would immediately hang with the `master` branch
of the GC, waiting on `GC.lock_write`: the GC is trying to collect
while Crystal is initializing constants and class variables
(`__crystal_main`) and it would lock the rwlock for write twice, never
unlocking it, before the start callback was registered, but not the
before collect callback (oops).

This patch:

1. Registers the GC callbacks right in the GC.init method; this makes
   sure all callbacks are registered before we start running any Crystal
   code.

2. Initializes the rwlock inside the GC.init method; this avoids a lazy
   initializer and makes sure the memory is properly initialized ASAP.

src/gc/boehm.cr

@@ -175,7 +175,7 @@ end
 
 module GC
   {% if flag?(:preview_mt) %}
-    @@lock = Crystal::RWLock.new
+    @@lock = uninitialized Crystal::RWLock
   {% end %}
 
   # :nodoc:
@@ -205,10 +205,33 @@ module GC
     {% end %}
     LibGC.init
 
+    {% if flag?(:preview_mt) %}
+      @@lock = Crystal::RWLock.new
+    {% end %}
+
     LibGC.set_start_callback -> do
       GC.lock_write
     end
 
+    # pushes the stack of pending fibers when the GC wants to collect memory:
+    {% unless flag?(:interpreted) %}
+      GC.before_collect do
+        Fiber.unsafe_each do |fiber|
+          fiber.push_gc_roots unless fiber.running?
+        end
+
+        {% if flag?(:preview_mt) %}
+          Thread.unsafe_each do |thread|
+            if fiber = thread.current_fiber?
+              GC.set_stackbottom(thread.gc_thread_handler, fiber.@stack_bottom)
+            end
+          end
+        {% end %}
+
+        GC.unlock_write
+      end
+    {% end %}
+
     {% if flag?(:tracing) %}
       if ::Crystal::Tracing.enabled?(:gc)
         set_on_heap_resize_proc
@@ -462,25 +485,6 @@ module GC
     end
   end
 
-  # pushes the stack of pending fibers when the GC wants to collect memory:
-  {% unless flag?(:interpreted) %}
-    GC.before_collect do
-      Fiber.unsafe_each do |fiber|
-        fiber.push_gc_roots unless fiber.running?
-      end
-
-      {% if flag?(:preview_mt) %}
-        Thread.unsafe_each do |thread|
-          if fiber = thread.current_fiber?
-            GC.set_stackbottom(thread.gc_thread_handler, fiber.@stack_bottom)
-          end
-        end
-      {% end %}
-
-      GC.unlock_write
-    end
-  {% end %}
-
   # :nodoc:
   def self.stop_world : Nil
     LibGC.stop_world_external