libosv.so: Add missing weak symbols

We define a lot of symbols using weak_alias(). Unfortunately,
scripts/libosv.py only copied global-binding symbols, not those with weak
binding. We also need to copy those with weak binding.

Note that they are not currently marked as weak symbols on the resulting
libosv.so. This will not make a practical difference, because OSv doesn't
respect the weakness of symbols anyway.

Fixes #365.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>