cmd/dist: avoid darwin_amd64 assumption in debug prints

Noted in CL 73212 review by crawshaw.
Neglected to update CL 73212 before submitting.

Also fix printing of target goos/goarch for cross-compile build.

Change-Id: If702f23071a4456810f1de6abb9115b38933c5c1
Reviewed-on: https://go-review.googlesource.com/74631
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/build.go

@@ -1155,7 +1155,7 @@ func cmdbootstrap() {
 	goInstall(toolchain...)
 	if debug {
 		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
-		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/../../darwin_amd64/runtime/internal/sys.a", tooldir))
+		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
 		copyfile(pathf("%s/compile2", tooldir), pathf("%s/compile", tooldir), writeExec)
 	}
 
@@ -1183,7 +1183,7 @@ func cmdbootstrap() {
 	goInstall(append([]string{"-a"}, toolchain...)...)
 	if debug {
 		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
-		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/../../darwin_amd64/runtime/internal/sys.a", tooldir))
+		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
 		copyfile(pathf("%s/compile3", tooldir), pathf("%s/compile", tooldir), writeExec)
 	}
 	checkNotStale(goBootstrap, append(toolchain, "runtime/internal/sys")...)
@@ -1212,19 +1212,19 @@ func cmdbootstrap() {
 		if vflag > 0 {
 			xprintf("\n")
 		}
-		xprintf("Building packages and commands for target, %s/%s.\n", goos, goarch)
 		goos = oldgoos
 		goarch = oldgoarch
 		os.Setenv("GOOS", goos)
 		os.Setenv("GOARCH", goarch)
 		os.Setenv("CC", defaultcctarget)
+		xprintf("Building packages and commands for target, %s/%s.\n", goos, goarch)
 	}
 	goInstall("std", "cmd")
 	checkNotStale(goBootstrap, "std", "cmd")
 	checkNotStale(cmdGo, "std", "cmd")
 	if debug {
 		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
-		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/../../darwin_amd64/runtime/internal/sys.a", tooldir))
+		run("", ShowOutput|CheckExit, pathf("%s/buildid", tooldir), pathf("%s/pkg/%s_%s/runtime/internal/sys.a", goroot, goos, goarch))
 		checkNotStale(goBootstrap, append(toolchain, "runtime/internal/sys")...)
 		copyfile(pathf("%s/compile4", tooldir), pathf("%s/compile", tooldir), writeExec)
 	}