cmd/link: remove all constants of elf

Use debug/elf instead.

Related:
CL 252478
CL 265317

Change-Id: If63b0458d9a6e825b40616bfb7a5a2c2e32402b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/265318


Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Ian Lance Taylor <iant@golang.org>