Make v8 snapshots work in debug-and-release mode

Make v8 snapshots work in debug-and-release mode

Debug snapshots and release snapshots aren't compatible. Both a
debug version and release version of the mkv8snapshot tool must be
built, and the corresponding executable selected when building v8.

Adopt the library naming convention for naming the mkv8snapshot
executable ("mkv8snapshot" in release, "mkv8snapshot_debug" in debug
on Mac, "mkv8snapshotd" in debug on Windows).

Change-Id: I7a94b09e7db7ed8bbaa293637c092a1d1d1dbaba
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>