This makes it possible for cmd/compile, when run with -dynlink on
darwin/amd64, to generate TLS_LE relocations which the linker then
turns into the appropriate PC-relative GOT load.

Change-Id: I1a71da432608bdb108ff66c22de600100209c873
Reviewed-on: https://go-review.googlesource.com/29393Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>