cmd/internal/ld: cache file name construction in linkgetline

This avoids repeated allocation and map lookups
when constructing the pcln tables.

For 6g compiling cmd/internal/gc/*.go this saves about 8% wall time.

Change-Id: I6a1a80e278ae2c2a44bd1537015ea7b4e7a4d6ca
Reviewed-on: https://go-review.googlesource.com/6793
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/internal/obj/link.go

@@ -154,14 +154,6 @@ type Auto struct {
 	Gotype  *LSym
 }
 
-type Hist struct {
-	Link    *Hist
-	Name    string
-	Line    int32
-	Offset  int32
-	Printed uint8
-}
-
 type Link struct {
 	Thechar            int32
 	Thestring          string