Issue title: Setting GOARCH
 Issue body: I am using gocode in vscode and I am trying to build a CGO project that requires GOARCH=386.
I cannot for the life of me figure out how to tell gocode to use this GOARCH.

No matter what I try, I get:
```
2017/05/28 23:44:22 Gocode's build context is:
2017/05/28 23:44:22  GOROOT: C:\Go\
2017/05/28 23:44:22  GOPATH: C:\GoProjects
2017/05/28 23:44:22  GOOS: windows
2017/05/28 23:44:22  GOARCH: amd64
2017/05/28 23:44:22  BzlProjectRoot: ""
2017/05/28 23:44:22  GBProjectRoot: ""
2017/05/28 23:44:22  lib-path: ""
2017/05/28 23:44:22 extracted expression tokens: window
2017/05/28 23:44:22 Offset: 0
2017/05/28 23:44:22 Number of candidates found: 0
2017/05/28 23:44:22 Candidates are:
```
 Comments: 
 Comment 0: Gocode passes environment variables from a client to a server. You should set in on whoever invokes the client and hope that it passes env vars to gocode. In your case that would be vscode, have no idea how it invokes gocode.
 Comment 1: I guess it works through https://marketplace.visualstudio.com/items?itemName=lukehoban.Go
 Comment 2: BTW vscode-go unsets GOARCH env:
https://github.com/Microsoft/vscode-go/blob/bca4dd5f31f32ac49da79580c07b4000f06287a3/src/goSuggest.ts#L129-L132
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