Rather than manually setting the LinkArch for each case, pass the correct
*obj.LinkArch to the arch* function, as is already done for archX86().

Change-Id: I4cf950780aa30a1385e785fb1d26edacb99bda79
Reviewed-on: https://go-review.googlesource.com/c/go/+/193818Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>