[Libomptarget] Do not run CPU tests if FFI was not found

Summary:
The previous behaviour before I made it dynamically open libFFI was that
these tests would be ignored if FFI was not found. This now allows tests
to be run without the dependency and thus the tests fails on some
buildbots. This simply makesit not build the tests if it's not present.

openmp/libomptarget/plugins-nextgen/CMakeLists.txt

@@ -82,11 +82,16 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "${tmachine}$")
   target_include_directories("omptarget.rtl.${tmachine_libname}" PRIVATE
                              ${LIBOMPTARGET_INCLUDE_DIR})
 
-  list(APPEND LIBOMPTARGET_TESTED_PLUGINS "omptarget.rtl.${tmachine_libname}")
-  set(LIBOMPTARGET_TESTED_PLUGINS
-      "${LIBOMPTARGET_TESTED_PLUGINS}" PARENT_SCOPE)
-  set(LIBOMPTARGET_SYSTEM_TARGETS 
-      "${LIBOMPTARGET_SYSTEM_TARGETS} ${tmachine_triple} ${tmachine_triple}-LTO" PARENT_SCOPE)
+  if(LIBOMPTARGET_DEP_LIBFFI_FOUND)
+    list(APPEND LIBOMPTARGET_TESTED_PLUGINS "omptarget.rtl.${tmachine_libname}")
+    set(LIBOMPTARGET_TESTED_PLUGINS
+        "${LIBOMPTARGET_TESTED_PLUGINS}" PARENT_SCOPE)
+    set(LIBOMPTARGET_SYSTEM_TARGETS
+        "${LIBOMPTARGET_SYSTEM_TARGETS} ${tmachine_triple}
+        ${tmachine_triple}-LTO" PARENT_SCOPE)
+  else()
+    libomptarget_say("Not generating ${tmachine_name} tests. LibFFI not found.")
+  endif()
 else()
   libomptarget_say("Not building ${tmachine_name} NextGen offloading plugin: machine not found in the system.")
 endif()