On 64-bit architectures, an IntPtr-sized write of Name::kEmptyHashField
will properly initialize the relevant Int32-sized payload of
Name::kHashFieldOffset, as well as zero out the unused portion of the
slot.

Bug: v8:7065
Change-Id: I93113c9744377e76cd0d264568c49b31a1e822a1
Reviewed-on: https://chromium-review.googlesource.com/788530Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49668}