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>