cmd/ld: fix addstrdata for big-endian systems
cmd/ld: fix addstrdata for big-endian systems

LGTM=rsc
R=minux, rsc
CC=golang-codereviews
https://codereview.appspot.com/158280043

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