Add a section identifier for declaring a start function as an index into
the function table. (This could also be done as a decl flag on the
function, but don't feel strongly here, since we probably want to redo
this when adding an import/export section.)

The start function must accept no parameters. Its return value is
currently ignored.

R=binji@chromium.org,bradnelson@chromium.org
BUG=chromium:575167
LOG=Y

Review URL: https://codereview.chromium.org/1692173002

Cr-Commit-Position: refs/heads/master@{#33978}