cmd/dist: add BOOT_GO_LDFLAGS - counterpart of BOOT_GO_GCFLAGS

This allows passing custom LDFLAGS while building the bootstrapping
tool.

Afterwards, GO_LDFLAGS will be used as usual.

Change-Id: I1e224e3ce8bf7b2ce1ef8fec1894720338f04396
GitHub-Last-Rev: 17d40dc2dd2f0815331cb2f8de3445f86687cc45
GitHub-Pull-Request: golang/go#31298
Reviewed-on: https://go-review.googlesource.com/c/go/+/171037
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/build.go

@@ -198,6 +198,7 @@ func xinit() {
 	}
 
 	gogcflags = os.Getenv("BOOT_GO_GCFLAGS")
+	goldflags = os.Getenv("BOOT_GO_LDFLAGS")
 
 	cc, cxx := "gcc", "g++"
 	if defaultclang {
@@ -661,6 +662,9 @@ func runInstall(dir string, ch chan struct{}) {
 		if goos == "android" {
 			link = append(link, "-buildmode=pie")
 		}
+		if goldflags != "" {
+			link = append(link, goldflags)
+		}
 		link = append(link, "-o", pathf("%s/%s%s", tooldir, elem, exe))
 		targ = len(link) - 1
 	}
@@ -1265,7 +1269,7 @@ func cmdbootstrap() {
 	}
 
 	gogcflags = os.Getenv("GO_GCFLAGS") // we were using $BOOT_GO_GCFLAGS until now
-	goldflags = os.Getenv("GO_LDFLAGS")
+	goldflags = os.Getenv("GO_LDFLAGS") // we were using $BOOT_GO_LDFLAGS until now
 	goBootstrap := pathf("%s/go_bootstrap", tooldir)
 	cmdGo := pathf("%s/go", gobin)
 	if debug {