Restore support for cross-compiling CMake itself

In commit eb583b0a66 (cmake_path command: path management, 2020-07-23,
v3.19.0-rc1~216^2~1) we added a `try_run`.  In cross-compilation mode,
C++ features tests must avoid running tests if there is no emulator
defined.

Source/Checks/cm_cxx_features.cmake

@@ -81,7 +81,13 @@ if(CMake_HAVE_CXX_MAKE_UNIQUE)
 endif()
 cm_check_cxx_feature(unique_ptr)
 if (NOT CMAKE_CXX_STANDARD LESS "17")
-  cm_check_cxx_feature(filesystem TRY_RUN)
+  if (NOT CMAKE_CROSSCOMPILING OR CMAKE_CROSSCOMPILING_EMULATOR)
+    cm_check_cxx_feature(filesystem TRY_RUN)
+  else()
+    # In cross-compiling mode, it is not possible to check implementation bugs
+    # so rely only on conformance done by compilation
+    cm_check_cxx_feature(filesystem)
+  endif()
 else()
   set(CMake_HAVE_CXX_FILESYSTEM FALSE)
 endif()