The root register is not available in JS-to-Wasm functions, and
this was not reflected in the linkage. Similarily, it is not
available in C-to-Wasm functions.

Change-Id: I2dbfd06ef99d6f9b9940e9489f563441d9ebfabd
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/1256766
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56346}