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/4321Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>