cmd/link: write memprofile in legacy format for compilebench

compilebench depends on the legacy heap profile format to read the
allocation stats of build tools. We're adding a benchmark for the
linker to compilebench, so we need the linker to emit the heap profile
in the legacy format.

This is the linker equivalent of CL 35484, which did this for the
compiler.

Change-Id: I16ad60c4f0fd80b4b6d608a5677ebe04e1fb5e5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/176057


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>