Broke freebsd/amd64 due to exposure of a latent bug.

««« original CL description
cmd/ld: fix operator precedence

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/114420043
»»»

TBR=dfc
R=dave
CC=golang-codereviews
https://golang.org/cl/120630043