cmd/asm, cmd/link: use full objabi header

The cmd/link check of the objabi header was a bit lax because
historically the assembler has not included the full version string.
And the assembler didn't do that because it didn't have access to it:
that was buried inside the compiler.

But now that we have cmd/internal/objabi, all the tools have full
access to the expected string, and they can use it, which simplifies
the cmd/link consistency check.

Do that.

Change-Id: I33bd2f9d36c373cc3c32ff02ec6368365088b011
Reviewed-on: https://go-review.googlesource.com/c/go/+/312030


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>