Commit d0b8e7fb introduced some functionality to allocate additional
space in the function table, filled with an "undefined" function
signature. Use -1 instead of undefined_value() as the latter can have
the top bits cleared (which happens often for arm64) and causes
intermittent test failures.

BUG=

Review-Url: https://codereview.chromium.org/2105293002
Cr-Commit-Position: refs/heads/master@{#37393}