cmd/link: replace interface{} fields with concrete types

The LSym.Section and Section.Elfsect fields were defined as interface{} but
always had the same concrete type (*Section and *ElfShdr respectively) so just
define them with that type. Reduces size of LSym from 328 to 320 bytes and
reduces best-of-10 maxresident size from 246028k to 238036k when linking
libstd.so.

Change-Id: Ie7112c53e4c2c7ce5fe233b81372aa5633f572e8
Reviewed-on: https://go-review.googlesource.com/10410
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -162,7 +162,7 @@ type Section struct {
 	Length  uint64
 	Next    *Section
 	Seg     *Segment
-	Elfsect interface{}
+	Elfsect *ElfShdr
 	Reloff  uint64
 	Rellen  uint64
 }