It removes the assertion that caused the bug report by introducing a new
trampoline (WasmBigIntToI64) for the conversion code stub between a Bigint
object and a Wasm i64 BigIntToI64).

The tests were updated to cover calling the stub from a Wasm callsite.

Bug: v8:8625
Change-Id: I55891001cfa72f6f2849792293b43bbb54147f1a
Reviewed-on: https://chromium-review.googlesource.com/c/1405028
Commit-Queue: Sven Sauleau <ssauleau@igalia.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58767}