cmd/go: build: add -static flag

This is a proposal to add -static flag to go build.

Producing a static build with go already requires a non-trivial amount of flags passed to go build, which on Linux currently amounts to something like:

-ldflags '-extldflags "-fno-PIC -static"' -buildmode pie -tags 'osusergo netgo static_build' [1]

...and this magic string keeps growing.

It would be awesome to encapsulate this sacred knowledge internally, exposing it via a new -static flag for go build and friends.

In addition to the above benefit of hiding the complexity, -static can also bring us more niceties, such as:

• automatic static build tag third-party software can rely upon. Currently using static_build tag seems like a de-facto standard, used a lot, but it has to be explicitly defined like in [1] above.
• providing --static flag to pkg-config invocations (those initiated by // #cgo pkg-config: lib lines in the source code). It will solve another issue (linking against a proper set of libraries for both static and dynamic link cases) for which a somewhat verbose workaround is currently required (for example, see ploop_link_static.go and ploop_link_dynamic.go).

See also

• https://github.com/golang/go/issues/12058 (original proposal for adding conditional --static to pkg-config)
• https://github.com/golang/go/issues/24787 (initial version of this very proposal)