[dev.power64] liblink: fix printing of branch targets

Print PC stored in target Prog* of branch instructions when
available instead of the offset stored in the branch
instruction.  The offset tends to be wrong after code
transformations, so previously this led to confusing listings.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/168980043
[dev.power64] test: disable nilptr3 test on power64x

The remaining failures in this test are because of incomplete
optimization support on power64x.  Tracked in issue 9058.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/168130043
[dev.power64] test: "fix" live.go test on power64x

On power64x, this one line in live.go reports that t is live
because of missing optimization passes.  This isn't what this
test is trying to test, so shuffle bad40 so that it still
accomplishes the intent of the test without also depending on
optimization.

LGTM=rsc
R=rsc, dave
CC=golang-codereviews
https://codereview.appspot.com/167110043