cmd/internal/gc, cmd/internal/obj: clean up string returns

An artifact of the c2go translation was
a handful of instances of code like:

var s string
s += "foo"
return s

This CL converts those to simply 'return "foo"'.

The conversion was done mechanically with the
quick-and-dirty cleanup script at
https://gist.github.com/josharian/1fa4408044c163983e62.

I then manually moved a couple of comments in fmt.go.

toolstash -cmp thinks that there are no functional changes.

Change-Id: Ic0ebdd10f0fb8de0360a1041ce5cd10ae1168be9
Reviewed-on: https://go-review.googlesource.com/6265Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/gc/mparith1.go

@@ -594,9 +594,8 @@ func Bconv(xval *Mpint, flag int) string {
 		p--
 		buf[p] = '-'
 	}
-	var fp string
-	fp += string(buf[p:])
-	return fp
+
+	return string(buf[p:])
 }
 
 func Fconv(fvp *Mpflt, flag int) string {

src/cmd/internal/obj/i386/list8.go

@@ -77,10 +77,7 @@ func Pconv(p *obj.Prog) string {
 }
 
 func Aconv(i int) string {
-	var fp string
-
-	fp += Anames[i]
-	return fp
+	return Anames[i]
 }
 
 var Register = []string{

src/cmd/internal/obj/x86/list6.go

@@ -89,10 +89,7 @@ func Pconv(p *obj.Prog) string {
 }
 
 func Aconv(i int) string {
-	var fp string
-
-	fp += Anames[i]
-	return fp
+	return Anames[i]
 }
 
 var Register = []string{