35223: Makefile: add a gss go build tag r=tbg a=mjibson

This allows us to only build gss support if the Makefile determines it's
ok, instead of relying on Go's build tags as well.

Release note: None

Co-authored-by: Matt Jibson <matt.jibson@gmail.com>
Co-authored-by: David Taylor <tinystatemachine@gmail.com>