[cmake] Compile everything with visibility hidden

Summary: This require to update cmake requirement to 3.5, which is the version that ship in xenial, so we are good.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: matiu, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1275