Adds the counter to x64 only.

Bug: v8:6900
Change-Id: Ia290102b38f029a0b71c40e4b00ecc5f07dfa59c
Reviewed-on: https://chromium-review.googlesource.com/704678
Commit-Queue: Michał Majewski <majeski@google.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48378}