[dev.typeparams] cmd/compile: move def of comparable to end of predeclared slices

This avoids changing the export ABI.

Change-Id: I58950c1f4c21859d91d66d352b88e8c0972b5b8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/333164


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>