Fix CMAKE_HOST_SYSTEM_PROCESSOR detection on GNU/Hurd (#15147)

On Debian GNU/Hurd 'uname -p' prints "unknown", so use 'uname -m'
instead.
Help: Document the WINCE variable
Help: Document that the CHECK_* macros create cache variables

Otherwise callers may expect to be able to re-use result variables.
Merge topic 'fix-gnu-hurd-host-detection'

89d7a5a4 Fix CMAKE_HOST_SYSTEM_PROCESSOR detection on GNU/Hurd (#15147)
Merge topic 'doc-WINCE-var'

4b4555de Help: Document the WINCE variable
Merge topic 'doc-check-results-cached'

908c7439 Help: Document that the CHECK_* macros create cache variables