Dropped non-supoorted "--no-as-needed" flag from OMPT tests for macOS

The flag "--no-as-needed" is not recognized by the linker on macOS making the following tests fail:

ompt/loadtool/tool_available/tool_available.c
ompt/loadtool/tool_not_available/tool_not_available.c
This patch removes this flag for macOS and adds it only for Linux and Windows.
I tested it on Ubuntu 16.04 and macOS HighSierra, with Clang/LLVM 6.0.1 and OpenMP trunk.

This solution was also discussed in the OpenMP-dev mailing list.

Patch provided by Simone Atzeni

Differential Revision: https://reviews.llvm.org/D48888

llvm-svn: 336327