[dev.power64] liblink: fix lost branch target
[dev.power64] liblink: fix lost branch target

A recent commit lost the branch target in the really-big-stack
case of splitstack, causing an infinite loop stack preempt
case.  Revive the branch target.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/157790044

d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh