Minor cleanup: Instead of a cryptic memcpy, just use ReadUnalignedValue
and WriteUnalignedValue.
Also add DCHECKs to these helpers to ensure that they are only used for
trivially copyable types.

R=ahaas@chromium.org

Bug: v8:7310
Change-Id: Id5014a828573f8d13a6c3a5380eae2f377e8f130
Reviewed-on: https://chromium-review.googlesource.com/948544Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51740}