Help: CMAKE_APPLE_SILICON_PROCESSOR cannot be set in a toolchain file

`CMakeDetermineSystem` determines the host system information before
loading the toolchain file.

Issue: #22012

Help/variable/CMAKE_APPLE_SILICON_PROCESSOR.rst

@@ -8,8 +8,7 @@ CMake what architecture to use for :variable:`CMAKE_HOST_SYSTEM_PROCESSOR`.
 The value must be either ``arm64`` or ``x86_64``.
 
 The value of this variable should never be modified by project code.
-It is meant to be set by a toolchain file specified by the
-:variable:`CMAKE_TOOLCHAIN_FILE` variable, or as a cache entry
-provided by the user, e.g. via ``-DCMAKE_APPLE_SILICON_PROCESSOR=...``.
+It is meant to be set as a cache entry provided by the user,
+e.g. via ``-DCMAKE_APPLE_SILICON_PROCESSOR=...``.
 
 See also the :envvar:`CMAKE_APPLE_SILICON_PROCESSOR` environment variable.