gobex: Use gcc builtin instead of g_atomic

gobex: Use gcc builtin instead of g_atomic

g_atomic_* end up using G_STATIC_ASSERT, causing gcc 4.8 to yell due to
-Wunused-local-typedefs.

/usr/include/glib-2.0/glib/gmacros.h:162:53: error: typedef ‘_GStaticAssertCompileTimeAssertion_2’ locally defined but not used [-Werror=unused-local-typedefs]
 #define G_STATIC_ASSERT(expr) typedef char G_PASTE (_GStaticAssertCompileTimeAssertion_, __COUNTER__)[(expr) ? 1 : -1]

Most of the uses of atomic operations were wrong. They were fixed as
well. If we are using atomic operations, reading the variable again
later for logging is not an option, we should use the return of the
atomic function used to fetch the variable.