Instrumented libraries: introduce release-specific patches.

This CL enables applying different custom patches depending on the Ubuntu
release (Precise/Trusty). It also adds two Trusty-specific patches.

BUG=414189
R=glider@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/614903003

Cr-Commit-Position: refs/heads/master@{#297672}