Make values which are referenced via ExternalReference constexpr, and
initialize them statically. This avoids dynamic initialization and
protects them against being overwritten from generated code.

R=neis@chromium.org

Bug: v8:7570
Change-Id: I1c6c10fbffea12dc1f5bf726313bf8388e6530a1
Reviewed-on: https://chromium-review.googlesource.com/1013518Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52633}