cmd/link: do not store compilation directory in DWARF info

This makes 'go install cmd/compile' in one directory produce
a different binary from running it in another directory,
which is problematic for reproducible builds.

Change-Id: If26685d2e45d2695413b472142b49694716575fa
Reviewed-on: https://go-review.googlesource.com/74790
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/link/internal/ld/dwarf.go

@@ -20,7 +20,6 @@ import (
 	"cmd/link/internal/sym"
 	"fmt"
 	"log"
-	"os"
 	"strings"
 )
 
@@ -1024,9 +1023,8 @@ func putpclcdelta(linkctxt *Link, ctxt dwarf.Context, s *sym.Symbol, deltaPC uin
  */
 
 func getCompilationDir() string {
-	if dir, err := os.Getwd(); err == nil {
-		return dir
-	}
+	// OS X requires this, but it's really none of its business.
+	// Hard-code "/" for reproducible builds.
 	return "/"
 }