cmd/go: use platform.MustLinkExternal in externalLinkingForced
This is a roll-forward of CL 477395 which was rolled back in CL 477736.
The earlier CL failed because we didn't account for the fact that
on some targets PIE is the default. That is now fixed.
Change-Id: I3e93faa9506033d27040cc9920836f010e05cd26
Reviewed-on: https://go-review.googlesource.com/c/go/+/477919 Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>