lang/v8, lang/v8-devel: Backport CVE fix

This fix has been backported instead of upgrading to a newer release as
the upstream release process is a complicated fast-moving target and the
current ports are using custom snapshots created by the port maintainer.

This will also limit the amount of potential fallout as we know the
existing v8 port works well enough to keep mongodb up to date.

PR: 201450
MFH: 2015Q3
Security: CVE-2015-5380
Security: 864e6f75-2372-11e5-86ff-14dae9d210b8