cmd/go: -buildmode=pie forces external linking mode on all systems

cmd/go: -buildmode=pie forces external linking mode on all systems

The go tool assumed that -buildmode=pie implied internal linking on
linux-amd64. However, that was changed by CL 36417 for issue #18968.

Fixes #21452

Change-Id: I8ed13aea52959cc5c53223f4c41ba35329445545
Reviewed-on: https://go-review.googlesource.com/57231
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Rob Pike <r@golang.org>