builder: add support for -x flag

Print commands as they are executed with the -x flag. This is not yet
complete: library builds don't print commands yet. But it's a step
closer.

builder/build.go

@@ -476,6 +476,9 @@ func Build(pkgName, outpath string, config *compileopts.Config, action func(Buil
 				}
 				ldflags = append(ldflags, dependency.result)
 			}
+			if config.Options.PrintCommands {
+				fmt.Printf("%s %s\n", config.Target.Linker, strings.Join(ldflags, " "))
+			}
 			err = link(config.Target.Linker, ldflags...)
 			if err != nil {
 				return &commandError{"failed to link", executable, err}