cmd/link: use HEADR to define FlagTextAddr (cosmetic change)

This cosmetic change defines ld.FlagTextAddr using ld.HEADR in
the Plan 9 cases, like it is done for other operating systems.

Change-Id: Ic929c1c437f25661058682cf3e159f0b16cdc538
Reviewed-on: https://go-review.googlesource.com/30912
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/arm64/obj.go

@@ -80,7 +80,7 @@ func archinit(ctxt *ld.Link) {
 		ld.HEADR = 32
 
 		if *ld.FlagTextAddr == -1 {
-			*ld.FlagTextAddr = 4128
+			*ld.FlagTextAddr = 4096 + int64(ld.HEADR)
 		}
 		if *ld.FlagDataAddr == -1 {
 			*ld.FlagDataAddr = 0

src/cmd/link/internal/x86/obj.go

@@ -79,7 +79,7 @@ func archinit(ctxt *ld.Link) {
 		ld.HEADR = 32
 
 		if *ld.FlagTextAddr == -1 {
-			*ld.FlagTextAddr = 4096 + 32
+			*ld.FlagTextAddr = 4096 + int64(ld.HEADR)
 		}
 		if *ld.FlagDataAddr == -1 {
 			*ld.FlagDataAddr = 0