cmd/link: reenable internal linking PIE on linux/amd64 and linux/arm64

cmd/link: reenable internal linking PIE on linux/amd64 and linux/arm64

It was enabled in CL 207877, but then accidentally disabled in
CL 207299 due to a bad rebase. Reenable.

Change-Id: I147bf724a4263d4aae54576a36cc7e1cad5e8a2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/221098
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>