gatomic: introduce G_ATOMIC_LOCK_FREE

We clean up the detection of if we should do 'real' atomic operations or
mutex-emulated ones with the introduction of a new (public) macro:
G_ATOMIC_LOCK_FREE.  If defined, our atomic operations are guaranteed to
be done in hardware.

We need to use __GCC_HAVE_SYNC_COMPARE_AND_SWAP_4 to determine if our
compiler supports GCC-style atomic operations from the gatomic.h header
because we might be building a program against GLib using a different
set of compiler options (or a different compiler) than was used to build
GLib itself.

Unfortunately, this macro is not available on clang, so it has currently
regressed to using the mutex emulation.  A bug about that has been
opened here:

  http://llvm.org/bugs/show_bug.cgi?id=11174

docs/reference/glib/glib-overrides.txt

@@ -1,6 +1,11 @@
 # This file makes most of the thread related macros look like
 # functions, which they really were, if possible easy.
 
+<MACRO>
+<NAME>G_ATOMIC_LOCK_FREE</NAME>
+#define G_ATOMIC_LOCK_FREE
+</MACRO>
+
 # default thread implementation
 
 <MACRO>

docs/reference/glib/glib-sections.txt

@@ -806,6 +806,9 @@ g_async_queue_sort_unlocked
 <SECTION>
 <TITLE>Atomic Operations</TITLE>
 <FILE>atomic_operations</FILE>
+G_ATOMIC_LOCK_FREE
+
+<SUBSECTION>
 g_atomic_int_get
 g_atomic_int_set
 g_atomic_int_inc