Add test_atomic_ops to perform minimal testing of used atomic primitives

Add test_atomic_ops to perform minimal testing of used atomic primitives

The main purpose of test_atomic_ops is to be able to check whether
it is safe to use compiler atomic intrinsics (enabled by defining
GC_BUILTIN_ATOMIC macro).

* tests/test_atomic_ops.c: New file.
* tests/tests.am [THREADS] (TESTS, check_PROGRAMS): Add
test_atomic_ops.
* tests/tests.am [THREADS] (test_atomic_ops_SOURCES,
test_atomic_ops_LDADD): New variable.
* tests/tests.am (check-without-test-driver): Run test_atomic_ops (if
available).