Ensure that all fields of `SyntheticModule` are set before creating
the exports hash table for it, because the latter may trigger
garbage collection, leading to crashes.

This has been causing failures in the Node.js CI over the last weeks,
after making the creating of synthetic modules part of Node’s
startup sequence.

(I am generally not very familiar with this part of the V8
code and there might be a better way, or possibly a way to add a
reliable regression test, that I am not aware of.)

Refs: https://github.com/nodejs/node/issues/30498
Refs: https://github.com/nodejs/node/issues/30648
Change-Id: I32da4b7bd888c6ec1421f34f5bd52e7bad154c1e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1939752
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65247}