cmd/link: fix suspicious code in emitPcln

cmd/link: fix suspicious code in emitPcln

In cmd/link/internal/ld/pcln.go:emitPcln, the code and the
comment don't match. I think the comment is right. Fix the code.

As a consequence, on Linux/AMD64, internal linking with PIE
buildmode with cgo (at least the cgo packages in the standard
library) now works. Add a test.

Change-Id: I091cf81ba89571052bc0ec1fa0a6a688dec07b04
Reviewed-on: https://go-review.googlesource.com/c/go/+/166017
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>