cmd/internal/obj: separate code layout from object writing

This will allow the compiler to crunch Prog lists down to code as each
function is compiled, instead of waiting until the end, which should
reduce the working set of the compiler. But not until Go 1.7.

This also makes it easier to write some machine code output tests
for the assembler, which is why it's being done now.

For #13822.

Change-Id: I0811123bc6e5717cebb8948f9cea18e1b9baf6f7
Reviewed-on: https://go-review.googlesource.com/18311
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/link.go

@@ -610,6 +610,12 @@ type Link struct {
 	Version            int
 	Textp              *LSym
 	Etextp             *LSym
+
+	// state for writing objects
+	Text  *LSym
+	Data  *LSym
+	Etext *LSym
+	Edata *LSym
 }
 
 // The smallest possible offset from the hardware stack pointer to a local