cmd/link: start at address 0 when external linking

When external linking, we are creating an object file, instead of
a executable. The absolute address is irrelevant. The external
linker will set it up. Start at address 0.

Change-Id: I3a2e0b8087b328d5c3144f29ca8ba6311aa39cba
Reviewed-on: https://go-review.googlesource.com/c/go/+/319830


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>