When compiling third-party projects with -Wbad-function-cast, the inline
g_atomic_pointer_get() implementation which uses C11 __atomic_load*()
calls on GCC was causing compilation errors like:

   error: cast from function call of type ‘long unsigned int’ to non-matching type ‘void *’

While we don’t want to compile all of GLib with -Wbad-function-cast, we
should support its headers being included in projects which do enable
that warning.

It doesn’t seem to be possible to cast away the warning (e.g. by casting
the function’s result through (void)), so we have to assign to an
intermediate integer of the right size first.

The same has to be done for the bool return value from
__sync_bool_compare_and_swap(). In that case, casting from bool to
gboolean raises a -Wbad-function-cast warning, since gboolean is
secretly int.

The atomic tests have been modified to enable -Wbad-function-cast to
catch regressions of this in futur...