cmd/cgo: use strings.Builder to build long strings
Change-Id: I33ba7c8a7b731647cdff3ffe7c4274f76f8923ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/638736 Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Jorropo <jorropo.pgm@gmail.com>