tst-mmap.so: Switch to C++ APIs

Use C++ APIs to remove dependency to OSv internals so that we can run
the same test on Linux. Make the concurrency tests depend on OSv because
C++ doesn't have a API for binding threads to specific CPUs and we don't
want to clutter the test with pthread'isms.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>