OpenBSD: Use 'arch -s' for host processor (#12143)

On OpenBSD 'uname -p' returns a long descriptive name for the processor.
Use 'arch -s' instead to get a short cpu name.

Suggested-by: Stuart Henderson

Modules/CMakeDetermineSystem.cmake

@@ -50,14 +50,17 @@ IF(CMAKE_HOST_UNIX)
     IF(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*")
       EXEC_PROGRAM(uname ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)
-    ELSE(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*")
+    ELSEIF(CMAKE_HOST_SYSTEM_NAME MATCHES "OpenBSD")
+      EXEC_PROGRAM(arch ARGS -s OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
+        RETURN_VALUE val)
+    ELSE()
       EXEC_PROGRAM(uname ARGS -p OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)
       IF("${val}" GREATER 0)
         EXEC_PROGRAM(uname ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
           RETURN_VALUE val)
       ENDIF("${val}" GREATER 0)
-    ENDIF(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*")
+    ENDIF()
     # check the return of the last uname -m or -p 
     IF("${val}" GREATER 0)
         SET(CMAKE_HOST_SYSTEM_PROCESSOR "unknown")