Indirect calls need a scratch register to load the target address. On
ia32 there's no easily-available scratch register.

This removes invalid code and documents a potential solution. But
ideally, this will remain unreachable since all inter-builtin calls
will be pc-relative.

Bug: v8:6666
Change-Id: I19e0ac699ee4757e3d5ec130b3e34a67cd1f851c
Reviewed-on: https://chromium-review.googlesource.com/1230096Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55999}