cmd/compile: allow fieldtrack of unexported fields

The fieldtrack support is experimental and used mainly inside Google,
where we have included this change for years. No reason not to make
it in the public copy.

Change-Id: I5233e4e775ccce60a17098c007aed8c82a0425d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/387355


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>