[dev.link] cmd/compile, cmd/asm: use bio.Writer for object file writing

It is convenient to have a seekable writer. A later CL will make
use of Seek.

Change-Id: Iba0107ce2975d9a451d97f16aa91a318dd4c90e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/196028
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/asm/main.go

@@ -46,12 +46,11 @@ func main() {
 	architecture.Init(ctxt)
 
 	// Create object file, write header.
-	out, err := os.Create(*flags.OutputFile)
+	buf, err := bio.Create(*flags.OutputFile)
 	if err != nil {
 		log.Fatal(err)
 	}
-	defer bio.MustClose(out)
-	buf := bufio.NewWriter(bio.MustWriter(out))
+	defer buf.Close()
 
 	if !*flags.SymABIs {
 		fmt.Fprintf(buf, "go object %s %s %s\n", objabi.GOOS, objabi.GOARCH, objabi.Version)
@@ -91,9 +90,8 @@ func main() {
 		} else {
 			log.Print("assembly failed")
 		}
-		out.Close()
+		buf.Close()
 		os.Remove(*flags.OutputFile)
 		os.Exit(1)
 	}
-	buf.Flush()
 }

src/cmd/compile/internal/gc/obj.go

@@ -166,7 +166,7 @@ func dumpLinkerObj(bout *bio.Writer) {
 
 	fmt.Fprintf(bout, "\n!\n")
 
-	obj.WriteObjFile(Ctxt, bout.Writer, myimportpath)
+	obj.WriteObjFile(Ctxt, bout, myimportpath)
 }
 
 func addptabs() {

src/cmd/internal/obj/objfile.go

@@ -8,6 +8,7 @@ package obj
 
 import (
 	"bufio"
+	"cmd/internal/bio"
 	"cmd/internal/dwarf"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
@@ -80,7 +81,8 @@ func newObjWriter(ctxt *Link, b *bufio.Writer, pkgpath string) *objWriter {
 	}
 }
 
-func WriteObjFile(ctxt *Link, b *bufio.Writer, pkgpath string) {
+func WriteObjFile(ctxt *Link, bout *bio.Writer, pkgpath string) {
+	b := bout.Writer
 	w := newObjWriter(ctxt, b, pkgpath)
 
 	// Magic header