Instead, cmd/compile can directly emit R_USEFIELD relocations.

Manually verified rsc.io/tmp/fieldtrack still passes.

Change-Id: Ib1fb5ab902ff0ad17ef6a862a9a5692caf7f87d1
Reviewed-on: https://go-review.googlesource.com/37871
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>