cmd/internal/obj: move the "unary destination" tables from asm to obj/*

Have the implementations of each architecture declare the one-operand,
destination-writing instructions instead of splitting the information between
there and asm.

Change-Id: I44899435011a4a7a398ed03c0801e9f81cc8c905
Reviewed-on: https://go-review.googlesource.com/6490
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/link.go

@@ -250,6 +250,7 @@ type LinkArch struct {
 	Assemble   func(*Link, *LSym)
 	Follow     func(*Link, *LSym)
 	Progedit   func(*Link, *Prog)
+	UnaryDst   map[int]bool // Instruction takes one operand, a destination.
 	Minlc      int
 	Ptrsize    int
 	Regsize    int