reduce impact on build cache when xgo upgrades

[xhd2015]

Currently xgo identifies its version by 2 constants: VERSION(string) and REVISION(number).

It will invalidate xgo's cache of older version, and each time it does so, it causes initial build time cost much longer.

However, now xgo's core becomes much more stable, it's unnecessary to invalidate cache when the core does not change at all.

Hence here proposing new 2 constants: CORE_VERSION and CORE_REVISION, it replaces VERSION and REVISION in these 2 cases:

• control whether the cache should be invalidated
• match the github.com/xhd2015/xgo/runtime's version

[xhd2015]

Amend the revision command: xgo revision --core will prints the core revision.