runtime, time: add API BuildTimestamp to report an application's build time

[vipally]

refer golang#23175

Use build timestamp to distinguish different version of application binary is a common way in other languages.
eg: C has macros "__DATE__" and "__TIME__" to fetch compiler time.
But golang has no way to achive this feature nowadays.
I have made a patch to provide fllow API to achive it.

• func runtime.BuildTimestamp() int64
• func time.BuildTime() time.Time

runtime.BuildTimestamp defines a buildtimestamp value which will be written by linker(cmd/link).

time.BuildTime() converts runtime.BuildTimestamp() to time.Time.
The expected usage of BuildTimestamp API is as follow:

func main() {
	println("hello world!")
	println("buildtimestamp:", runtime.BuildTimestamp())
	println("buildtime     :", time.BuildTime().String())
	// Output:
	// hello world!
	// buildtimestamp: 1513671919
	// buildtime     : 2017-12-19 16:25:19 +0800 CST
}

It will be my pleasure that if go team can accept this feature in go repository.
The branch

[vipally]

This issue has been reject by offical
see golang#23175
So I provide my own go master branch to support some useful issues that official repository do not support

[vipally]

runtime.UnixNow
runtime.UnixNowNano
do not include in this release