Use V8 Code caching when host_arch == target_arch. This commit may
slightly improve startup time, and definitely improves require times
(when doing many of them).

For example, executing following file:

var fs = require('fs');
var http = require('http');
var https = require('https');
var path = require('path');
var util = require('util');

Takes: ~74ms on master
Takes: ~54ms with this commit

Number were taken on OS X with 64-bit node.js binaries.

cc @nodejs/collaborators @nodejs/v8