Implement build and test using mage

[otis]

Summary

While working on new sysutils/beats7, I found out that it is now being built using devel/mage.

This proposed change implements new argument for USES=go (mage)

[otis]

Reduce diff

[mat]

Could you use devel/arcanist, or at least generate a diff with full context like it does, with svn diff -x -U9999 or git diff -U9999.

Mk/Uses/go.mk

143–150

In USES, we try to follow the order of stuff in a port's Makefile, so variable definitions should happen before targets start to be defined, so, just before the post-extract target.

179–184

Same for here.

[otis]

• diff with full context
• reordered statements

[mat]

Mk/Uses/go.mk

138–145

We usually write VARIABLE=<tab> to let code breathe, it makes it more readable for the brain by pre-splitting logical blocks of text.

141–143

You should probably put all this one one line, it would probably easier to read.

[dmgk]

I think the decision was to put this on hold unless more Go ports use mage for build.