cmd/dist: set the default external linker on platforms without gcc

The go tool already sets -extld to the appropriate compiler. This
CL changes cmd/dist to do the same, to fix bootstrapping on platforms
that only have clang (Android and iOS).

Updates #31722

Change-Id: I8a4fd227f85a768053a8946198eab68bbbdf9ae5
Reviewed-on: https://go-review.googlesource.com/c/go/+/174305
Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/dist/build.go

@@ -666,6 +666,7 @@ func runInstall(dir string, ch chan struct{}) {
 		if goldflags != "" {
 			link = append(link, goldflags)
 		}
+		link = append(link, "-extld="+compilerEnvLookup(defaultcc, goos, goarch))
 		link = append(link, "-o", pathf("%s/%s%s", tooldir, elem, exe))
 		targ = len(link) - 1
 	}