cmd/go: add a way to support GOFLAGS=-ldflags='-w -s'

[mvdan]

The previous documentation example mentioned GOFLAGS='-ldflags=-w -ldflags=-s', which does something completely different. The second per-package flag overrides the first, so the linker ends up being called with just -s and not -w -s.

There should be a way to do go build -ldflags='-w -s' via GOFLAGS. This can either be done by tweaking GOFLAGS, or by tweaking per-package flags - I personally don't mind, as long as there's a way to achieve it.

/cc @bcmills @rsc

[gopherbot]

Change https://golang.org/cl/152479 mentions this issue: cmd/go: revert multi-flag GOFLAGS doc example

[bcmills]

#26849 is one possible mechanism.

[mvdan]

Go 1.11 supposedly shipped with this feature, so I'm milestoning this for 1.13 to "get that back". Within quotes because it never actually worked :)

[bcmills]

The underlying use-case is the same as #26849. Consolidating into that issue.