The fuzzer found a crash when we want to execute the {valueOf} function
of an imported value for an i64-global. The problem is that we cannot
execute JavaScript at that moment (I did not check why, I guess we open
some scope at some point). I checked the WebAssembly spec now, and it
defines that only numbers are valid values for imported globals. I
adjust our bigint implementation accordingly with this CL, i.e. that
only bigint values are valid as imported i64-globalsl.
I also created github issues to discuss this problem.

R=jkummerow@chromium.org

Bug: chromium:1001804
Change-Id: I47f0b31fab53163346f341ad290fd3c58e7707bf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1792167
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63621}