deps: backport b096c44 from upstream V8

Original commit message:

    [build] Introduce an embedder version string

    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 at 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 "6.0.287.53-emb.1", the embedder must set
    V8_EMBEDDER_STRING to "-emb.1".

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

    BUG=v8:5740
    R=machenbach@chromium.org

    Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
    Change-Id: Ifa2d9bd213795e6d54886436f8c3787ac6162823
    Reviewed-on: https://chromium-review.googlesource.com/690475


Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
    Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
    Cr-Commit-Position: refs/heads/master@{#48301}

Refs: https://github.com/v8/v8/commit/b096c44ffc32d84979bd2ea64ddfd08d643581ff
PR-URL: https://github.com/nodejs/node/pull/15785


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>