runtime: use fixed TLS offsets on darwin/amd64 and darwin/386

runtime: use fixed TLS offsets on darwin/amd64 and darwin/386

Fixes #23617

Note that this CL does not affect darwin/arm and darwin/arm64,
still TBD what, if anything, needs to be done for those.

This is a fix of CL 105975 which was reverted in CL 106155.
Needed to use movl instead of movq for 386.

Change-Id: I0db7f8087173869e60cc22c6c3124fa0a0739b46
Reviewed-on: https://go-review.googlesource.com/106156
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>