[release-branch.go1.8] cmd/link: use external linking for PIE by default

Now `go test -buildmode=pie std -short` passes on linux/amd64.

Updates #18968

Change-Id: Ide21877713e00edc64c1700c950016d6bff8de0e
Reviewed-on: https://go-review.googlesource.com/36417
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-on: https://go-review.googlesource.com/36421
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: David Crawshaw <[email protected]>
Reviewed-by: Minux Ma <[email protected]>

Author: crawshaw

src/cmd/link/internal/ld/config.go

@@ -238,6 +238,8 @@ func determineLinkMode(ctxt *Link) {
 				Linkmode = LinkExternal
 			} else if iscgo && externalobj {
 				Linkmode = LinkExternal
+			} else if Buildmode == BuildmodePIE {
+				Linkmode = LinkExternal // https://golang.org/issue/18968
 			} else {
 				Linkmode = LinkInternal
 			}