[Orc] Enable ORC execution unit tests on non-windows platforms.

Previously these were Darwin-only. Since the switch to direct binary emission
of stubs, trampolines and resolver blocks, these should work on other *nix
platforms too.

These tests can be enabled on Windows once known issues with ORC's handling of
Windows symbol mangling (see e.g. https://llvm.org/PR25940) have been fixed.

llvm-svn: 258031