cmd/compile, cmd/link: remove unused fields from relocations

Reduces size of archives in pkg/linux_amd64 by 3% from 41.5MB to 40.2MB

Change-Id: Id64ca7995de8dd84c9e7ce1985730927cf4bfd66
Reviewed-on: https://go-review.googlesource.com/20912Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/internal/goobj/read.go

@@ -635,9 +635,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 			rel.Size = r.readInt()
 			rel.Type = r.readInt()
 			rel.Add = r.readInt()
-			r.readInt() // Xadd - ignored
 			rel.Sym = r.readSymID()
-			r.readSymID() // Xsym - ignored
 		}
 
 		if s.Kind == STEXT {

src/cmd/internal/obj/objfile.go

@@ -70,9 +70,7 @@
 //	- siz [int]
 //	- type [int]
 //	- add [int]
-//	- xadd [int]
 //	- sym [symref index]
-//	- xsym [symref index]
 //
 // Each local has the encoding:
 //
@@ -492,9 +490,7 @@ func writesym(ctxt *Link, b *Biobuf, s *LSym) {
 		wrint(b, int64(r.Siz))
 		wrint(b, int64(r.Type))
 		wrint(b, r.Add)
-		wrint(b, 0) // Xadd, ignored
 		wrsym(b, r.Sym)
-		wrsym(b, nil) // Xsym, ignored
 	}
 
 	if s.Type == STEXT {

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

@@ -72,9 +72,7 @@ package ld
 //	- siz [int]
 //	- type [int]
 //	- add [int]
-//	- xadd [int]
 //	- sym [symref index]
-//	- xsym [symref index]
 //
 // Each local has the encoding:
 //
@@ -250,9 +248,7 @@ overwrite:
 			r.Siz = rduint8(f)
 			r.Type = rdint32(f)
 			r.Add = rdint64(f)
-			rdint64(f) // Xadd, ignored
 			r.Sym = rdsym(ctxt, f, pkg)
-			rdsym(ctxt, f, pkg) // Xsym, ignored
 		}
 	}