Sometimes, the embedder might want to merge a fix to an abandoned branch
or to a supported branch but the fix is not relevant to Chromium.
This adds a new version string that the embedder can set on compile time
and that will be appended to the official V8 version.
The separator must be provided in the string. For instance, to have a
full version string like "5.5.372.37.custom.1", the embedder must set
V8_EMBEDDER_STRING to ".custom.1".

Related Node.js issue: https://github.com/nodejs/node/pull/9754

BUG=v8:5740
R=machenbach@chromium.org,hablich@chromium.com,ofrobots@google.com

Review-Url: https://codereview.chromium.org/2619213002
Cr-Commit-Position: refs/heads/master@{#42175}