cmd/link/internal/ld: don't use linkname before 1.12
Before the 1.12 release the use of linkname did not prevent a compiler
error for an empty function body (see #23311). Add some build tags so
that cmd/link will build with earlier releases.
It's true that we currently require Go 1.16 as the bootstrap compiler (#44505).
But for this simple case keep things working with older compilers for now.
Change-Id: I67fe021406096c64c01d6e2c9adbcc4388988a6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/355690
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>