Always use /usr/local/share/zsh/site-functions

Summary:
On Linux, zsh site-functions directory is also
in /usr/local/share, and not in /usr/share.

Fixes the following issue:

----------
          ID: /usr/share/zsh/site-functions/_pm
    Function: file.managed
      Result: False
     Comment: Parent directory not present
     Started: 19:00:18.026241
    Duration: 530.975 ms
     Changes:

Test Plan: Test on Eglide

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2144