cmd/internal/obj: some platform independent bits of proper toolchain support for thread local storage

Also simplifies some silliness around making the .tbss section wrt internal
vs external linking. The "make TLS make sense" project has quite a few more
steps to go.

Issue #11270

Change-Id: Ia4fa135cb22d916728ead95bdbc0ebc1ae06f05c
Reviewed-on: https://go-review.googlesource.com/13990Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>