cmd/link: do not use GO_LDSO when cross compile

GO_LDSO is a setting that is set when the toolchain is build. It
only makes sense to use it on the host platform. Do not use it
when targetting a different platform.

In the past it was not a problem as GO_LDSO was almost always
unset. Now, with CL 301989 it is almost always set (maybe we want
to revisit it).

Fixes #47760.

Change-Id: I2704b9968781f46e2d2f8624090db19689b1a32f
Reviewed-on: https://go-review.googlesource.com/c/go/+/343010


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>