gthread: Rework to avoid holding a mutex half the time

This code was a persistent source of `-fsanitize=thread` errors
when I was trying to use it on OSTree.

The problem is that while I think this code is functionally correct,
we hold a mutex during the writes, but not the reads, and TSAN (IMO
correctly) flags that.

Reading this, I don't see a reason we need a mutex at all.  At the
cost of some small code duplication between posix/win32, we can just
pass the data we need down into each implementation.  This ends up
being notably cleaner I think than the awkward "lock/unlock to
serialize" dance.

(Minor review changes made by Philip Withnall <withnall@endlessm.com>.)

https://gitlab.gnome.org/GNOME/glib/issues/1224

glib/gthreadprivate.h

@@ -37,8 +37,11 @@ struct  _GRealThread
 /* system thread implementation (gthread-posix.c, gthread-win32.c) */
 void            g_system_thread_wait            (GRealThread  *thread);
 
-GRealThread *   g_system_thread_new             (GThreadFunc   func,
+GRealThread *   g_system_thread_new             (GThreadFunc   proxy,
                                                  gulong        stack_size,
+                                                 const char   *name,
+                                                 GThreadFunc   func,
+                                                 gpointer      data,
                                                  GError      **error);
 void            g_system_thread_free            (GRealThread  *thread);