cmd/compile/internal/abt: use strings.Builder

cmd/compile/internal/abt: use strings.Builder

Since go1.17 is now used for bootstrapping.

Change-Id: I5692ff4b38b7d9a279b20662ce5348ff34a2dde6
Reviewed-on: https://go-review.googlesource.com/c/go/+/427134
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>