x/tools/cmd/godoc: add way to select GOOS/GOARCH for syscall pkg

[techtonik]

https://golang.org/pkg/syscall/#pkg-variables
syscall documentation only shows docs for LInux, which makes it harder than necessary to troubleshoot cross-platform bugs
#12978 (comment)

[bradfitz]

This is super low priority since it basically only affects the syscall package, and people can just run godoc locally with GOOS= and GOARCH= set to see the syscall docs on another platform.

Maybe we could merge GOOS=windows and GOOS=linux symbols together in godoc by default for just the syscall package.

/cc @griesemer @adg

[techtonik]

@bradfitz do you have stats how often people run these commands? I am asking because I read and search for everything online.

[bradfitz]

We don't track stats.

[bradfitz]

Pending CL at https://go-review.googlesource.com/1371

[larzconwell]

https://go-review.googlesource.com/#/c/1371/ Has been merged so this can probably be closed now.

You can change the default GOOS/GOARCH using the query string in golang.org/pkg requests.