shared: rework _NM_GET_PRIVATE() to use _Generic()

_NM_GET_PRIVATE() used typeof() to propagate constness of the @self
pointer. However, that means, it could only be used with a self pointer
of the exact type. That means, you explicitly had to cast from (GObject *)
or from (void *).
The requirement is cumbersome, and often led us to either create @self
pointer we didn't need:

    NMDeviceVlan *self = NM_DEVICE_VLAN (device);
    NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE (self);

or casting:

    NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE ((NMDevice *) device);

In both cases we forcefully cast the source variable, loosing help from
the compiler to detect a bug.

For "nm-linux-platform.c", instead we commonly have a pointer of type
NMPlatform. Hence, we always forcefully cast the type via _NM_GET_PRIVATE_VOID().

Rework the macro to use _Generic(). If compiler supports _Generic(), then we
will get all compile time checks as desired. If the compiler doesn't support
_Generic(), it will still work. You don't get the compile-time checking of course,
but you'd notice that something is wrong once you build with a suitable
compiler.

src/platform/nm-linux-platform.c

@@ -2867,7 +2867,7 @@ struct _NMLinuxPlatformClass {
 
 G_DEFINE_TYPE (NMLinuxPlatform, nm_linux_platform, NM_TYPE_PLATFORM)
 
-#define NM_LINUX_PLATFORM_GET_PRIVATE(self) _NM_GET_PRIVATE_VOID(self, NMLinuxPlatform, NM_IS_LINUX_PLATFORM)
+#define NM_LINUX_PLATFORM_GET_PRIVATE(self) _NM_GET_PRIVATE2(self, NMLinuxPlatform, NM_IS_LINUX_PLATFORM, NMPlatform)
 
 NMPlatform *
 nm_linux_platform_new (gboolean log_with_ptr, gboolean netns_support)