[ELF] - Treat .openbsd.randomdata as relro section

That was requested by Mark Kettenis in llvm-dev:

"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."

He suggested code change, I added testcase.

Differential revision: https://reviews.llvm.org/D27974

llvm-svn: 290174