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>