[lld] Treat .openbsd.randomdata as read-only

It is the intention that .openbsd.randomdata sections are made
read-only after initialization. The native (ld.bfd based) OpenBSD
toolchain accomplishes this by including .openbsd.randomdata into the
PT_GNU_RELRO segment. The diff below makes ldd do the same.

Index: ELF/Writer.cpp

LGTM

From: Rui Ueyama <ruiu@google.com>
Date: Mon, 19 Dec 2016 20:08:59 -0600

LGTM

Great. I don't have commit access, so it would be great if you could
commit this change for me.

Just realized that that I should have sent this to llvm-commits
instead. Apologies for that.

I have some further fixes. Is Phabricator preferred these days for
submitting patches?

Thanks,

Mark