dri megadriver_stub: add compatibility for older DRI loaders

dri megadriver_stub: add compatibility for older DRI loaders

To help the transition period when DRI loaders are being updated
to support the newer __driDriverExtensions_foo mechanism,
we populate __driDriverExtensions with the extensions returned
by __driDriverExtensions_foo during a library contructor
function.

We find the driver foo's name by using the dladdr function
which gives the path of the dynamic library's name that
was being loaded.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Keith Packard <keithp@keithp.com>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>