Update Subzero to start parsing PNaCl bitcode files.

This patch only handles global addresses in PNaCl bitcode files.
Function blocks are still not parsed. Also, factors out a common API
for translation, so that generated ICE can always be translated using
the same code.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3892
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/361733002