debug/elf: rename Chdr64.Reserved to _

This future-proofs the Chdr64 structure against later versions of ELF
defining this field and declutters the documentation without changing
the layout of the struct.

This structure does not exist in the current release, so this change
is safe.

Change-Id: I239aad7243ddaf063a1f8cd521d8a50b30413281
Reviewed-on: https://go-review.googlesource.com/18028Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

api/go1.6.txt

@@ -135,7 +135,6 @@ pkg debug/elf, type Chdr32 struct, Size uint32
 pkg debug/elf, type Chdr32 struct, Type uint32
 pkg debug/elf, type Chdr64 struct
 pkg debug/elf, type Chdr64 struct, Addralign uint64
-pkg debug/elf, type Chdr64 struct, Reserved uint32
 pkg debug/elf, type Chdr64 struct, Size uint64
 pkg debug/elf, type Chdr64 struct, Type uint32
 pkg debug/elf, type CompressionType int

src/debug/elf/elf.go

@@ -2006,7 +2006,7 @@ type Dyn64 struct {
 // ELF64 Compression header.
 type Chdr64 struct {
 	Type      uint32
-	Reserved  uint32
+	_         uint32 /* Reserved. */
 	Size      uint64
 	Addralign uint64
 }