Fix CMAKE_HOST_SYSTEM_PROCESSOR detection on GNU/Hurd (#15147)

On Debian GNU/Hurd 'uname -p' prints "unknown", so use 'uname -m'
instead.

Modules/CMakeDetermineSystem.cmake

@@ -47,7 +47,7 @@ if(CMAKE_HOST_UNIX)
   if(CMAKE_UNAME)
     exec_program(uname ARGS -s OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_NAME)
     exec_program(uname ARGS -r OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION)
-    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin")
+    if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin|^GNU$")
       exec_program(uname ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)
       if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin" AND