liblink, cmd/6l: re-enable -shared on amd64

The creation of liblink and subsequent introduction of more explicit
TLS handling broke 6l's (unsupported) -shared flag.  This change adds
-shared flags to cmd/asm and 6g and changes liblink to generate shared-
library compatible instruction sequences when they are passed, and
changes 6l to emit the appropriate ELF relocation.

A proper fix probably also requires go tool changes.

Fixes #9652.

Change-Id: I7b7718fe7305c802ac994f4a5c8de68cfbe6c76b
Reviewed-on: https://go-review.googlesource.com/4321
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/asm/main.go

@@ -41,6 +41,9 @@ func main() {
 		ctxt.Debugasm = 1
 	}
 	ctxt.Trimpath = *flags.TrimPath
+	if *flags.Shared {
+		ctxt.Flag_shared = 1
+	}
 	ctxt.Bso = obj.Binitw(os.Stdout)
 	defer obj.Bflush(ctxt.Bso)
 	ctxt.Diag = log.Fatalf