[Solaris] Silence -pthread warning on Solaris

Summary:
During make check-all on Solaris, I see several instances of this warning:

clang-6.0: warning: argument unused during compilation: '-pthread' [-Wunused-command-line-argument]

Since Solaris 10, libpthread has been folded into libc, so there's nothing to do.  gcc
just ignores -pthread here.  Darwin claims the option to silence the warning, and
this patch follows that lead.

Reviewers: rsmith, fedor.sergeev

Reviewed By: fedor.sergeev

Subscribers: cfe-commits, fedor.sergeev

Differential Revision: https://reviews.llvm.org/D41242

llvm-svn: 324344