Autogen: Add clang --target= flag to compiler predefines command

Brad Kingrequested to merge
brad.king/cmake:autogen-moc-predef-cross-compile into master

Respect the compiler target value CMAKE_<LANG>_COMPILER_TARGET in CMAKE_<LANG>_COMPILER_PREDEFINES_COMMAND to get proper moc predefines during cross-compiling.

For Android, when we set CMAKE_<LANG>_COMPILER_TARGET to the Android target architecture, add it to CMAKE_<LANG>_COMPILER_PREDEFINES_COMMAND also. This is needed to make moc predefines aware of __ANDROID__.

Fixes: #18425 (closed)

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