Since we don't support gcc 2.96-4.0 any more, we can generalize the
V8_INFINITY macro to always use std::numeric_limits<double>::infinity().
This also makes value constexpr on all systems.

R=tebbi@chromium.org

Change-Id: Ifa97dd2ee6d2c1e179c45f60a82d1ea8481e0590
Reviewed-on: https://chromium-review.googlesource.com/725733Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48694}