cmd/link: mark PIE binaries as ET_DYN

Part of adding PIE internal linking on linux/amd64.

Change-Id: I586e7c2afba349281168df5e20d2fdcb697f6e37
Reviewed-on: https://go-review.googlesource.com/28542Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -2589,6 +2589,8 @@ elfobj:
 
 	if Linkmode == LinkExternal {
 		eh.type_ = ET_REL
+	} else if Buildmode == BuildmodePIE {
+		eh.type_ = ET_DYN
 	} else {
 		eh.type_ = ET_EXEC
 	}