This adds an attribute field to the binary encoding of exception types
in the exceptions and import section. Currently the attribute value is
not used and expected to be zero, but it ensures the binary encoding is
extensible for future changes.

R=clemensh@chromium.org
TEST=unittests/WasmModuleVerifyTest
BUG=v8:8153

Change-Id: I6f0e10cb1b6515177d8200ebf1f4f0b122832868
Reviewed-on: https://chromium-review.googlesource.com/c/1291075
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56841}