use xgo for cross compile

[arcolife]

Failed to produce darwin binary with hmake build, because a vendor dep uses libsecp256k1 which in turn depends on import "C" in golang from under secp256.go

screenshot courtesy: @hixichen

So now, we have 2 options:

1. Cross compile from linux to darwin https://github.com/karalabe/xgo
2. Compile natively

[hixichen]

What is our current status: we are packaging go binary into docker-based image and using runnint container to compile the code.

What is the issue:

The docker-based image is based on ubuntu, need cross compile for darwin since cgo was imported

[hixichen]

please check https://github.com/truechain/truechain-consensus-core/pull/62