Use `extern "C"` to declare the __readfsdword function.
Fixes error C2732 in the Node.js CI.

Bug: chromium:796644
Change-Id: If261985e65bfdade53ce06ff28afe0e2db402f7a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3086457Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Cr-Commit-Position: refs/heads/master@{#76259}