devel/go-tools: update to 0.15.0
- hook up the experimental internal/cmd/deadcode command
Selected upstream changes:
- cmd/compilebench: pass linker flags to prebuild
- cmd/present: drop NaCl reference from docs
- cmd/bundle: drop old +build lines
- go.mod: update golang.org/x dependencies
Changelog: https://github.com/golang/tools/compare/v0.14.0...v0.15.0