cmd/link: rename ELF struct fields to remove "elf" prefix

cmd/link: rename ELF struct fields to remove "elf" prefix

While we're here rename setupplt to setupPLT.

This is a pure naming change with no semantic change.

Change-Id: Ib0312fb6568475b620dab7632438b4d25e4d9cc0
Reviewed-on: https://go-review.googlesource.com/c/go/+/463744
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@golang.org>