Ensure that we can test libcu++ against architectures < 70 (#1475)

* Ensure that we can test libcu++ against architectures < 70

In our header test suite we compile headers that require a certain architecture only against those architectures that actually satisfy the requirement.

We missed the case when no architecture satisfy the constraints and we should not test that header at all.

Fixes nvbug4541374

* Use `NOT` instead of comparison with empty string

libcudacxx/test/public_headers/CMakeLists.txt

@@ -47,6 +47,10 @@ function(libcudacxx_add_public_header_test header)
   # Ensure that if this is an atomic header, we only include the right architectures
   string(REGEX MATCH "atomic|barrier|latch|semaphore|annotated_ptr|pipeline" match "${header}")
   if(match)
+    # Ensure that we only compile the header when we have some architectures enabled
+    if (NOT architectures_at_least_sm70)
+      return()
+    endif()
     set_target_properties(headertest_${header_name} PROPERTIES CUDA_ARCHITECTURES "${architectures_at_least_sm70}")
   endif()