wayland-egl: enhance the symbol test

The current test had a few fall-outs:
 - it was checking only for T (.text) symbols
 - did not consider symbol removal

Fix that by fetching all the symbols and doing a bidirectional check -
for added and removed symbols. Error out with informative message for
each case.

v2: Rebase on top of $NM patch.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>