cmd/6l: fix OpenBSD build
cmd/6l: fix OpenBSD build

Avoid generating TLS relocations on OpenBSD.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7641055

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang