cmd/ld: fix OpenBSD (third try)
cmd/ld: fix OpenBSD (third try)

On OpenBSD don't mark runtime.{g,m} as STT_TLS symbols.

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7867046

dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,apparentlymart/golang,apparentlymart/golang