Fix processing of global variable indices in the global vars block.

Fix processing of global variable indices in the global vars block.

The code used to use a vector to hold global variables associated with
indices. The problem was that the count record in the global vars
block would generate variables for the given count (even if very
large).

To fix this, we created a local unordered map to associate indices
with defined/referenced globals. After processing the global vars
block, this unordered map is used to verify the size makes sense, and
then install the recognized global variables into the (top-level)
contents.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4257
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1278793006 .