Consolidate some redundant definitions of ARRAY_SIZE() macro.

Previous to this patch, there were 13 identical definitions of this
macro in Mesa source.  That's ridiculous.  This patch consolidates 6
of them to a single definition in src/mesa/main/macros.h.

Unfortunately, I wasn't able to eliminate the remaining definitions,
since they occur in places that don't include src/mesa/main/macros.h:

- include/pci_ids/pci_id_driver_map.h
- src/egl/drivers/dri2/egl_dri2.h
- src/egl/main/egldefines.h
- src/gbm/main/backend.c
- src/gbm/main/gbm.c
- src/glx/glxclient.h
- src/mapi/mapi/stub.c

I'm open to suggestions as to how to deal with the remaining redundancy.