cmd/ld: attempt at fixing openbsd build
cmd/ld: attempt at fixing openbsd build

OpenBSD is excluded from all the usual thread-local storage
code, not just emitting the tbss section in the external link .o
but emitting a PT_TLS section in an internally-linked executable.
I assume it just has no proper TLS support. Exclude it here too.

TBR=iant
CC=golang-codereviews
https://codereview.appspot.com/87900045

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