[dev.unified] cmd/compile: restore Unified IR linkname pragma diagnostic

[dev.unified] cmd/compile: restore Unified IR linkname pragma diagnostic

CL 333109 restore the diagnostic for irgen, now it's safe to restore for
Unified IR, too.

Updates #53058

Change-Id: I467902c0e9fa451aaa78cf0813231f14d9d7a3a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/410346
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>