[docs] HowToCrossCompileLLVM.rst: update cmake options

This patch updates the cmake options suggested when cross compiling. This should fix [llvm#52819](llvm#52819).

Brad King (Member of CMake) says:

The linked [CMAKE_CROSSCOMPILING](https://cmake.org/cmake/help/v3.22/variable/CMAKE_CROSSCOMPILING.html) documentation says:

This variable will be set to true by CMake if the `CMAKE_SYSTEM_NAME` variable has been set manually (i.e. in a toolchain file or as a cache entry from the cmake command line).

It is not meant to be set by project code or toolchain files. It is always set automatically. Don't put `set(CMAKE_CROSSCOMPILING ON)` anywhere in your code.

`CMAKE_CROSSCOMPILING` indicates only whether `CMAKE_SYSTEM_NAME` was set by the user/project/toolchain-file instead of by CMake.

In LLVM project, `CMAKE_CROSSCOMPILING` is used to determine whether to execute some tests on the host machine.

LLVM needs to use another method for that. `CMAKE_CROSSCOMPILING` is not a reliable indicator of whether produced binaries will run on the host, and does not claim so in its documentation. If one sets `CMAKE_SYSTEM_NAME` to Linux in a toolchain file, and builds on a Linux host, that doesn't mean the target architecture or minimum glibc version is the same.

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D119804

Author: fourdim

llvm/docs/HowToCrossCompileLLVM.rst

@@ -40,14 +40,16 @@ see :doc:`CMake`.
 
 The CMake options you need to add are:
 
- * ``-DCMAKE_CROSSCOMPILING=True``
+ * ``-DCMAKE_SYSTEM_NAME=<target-system>``
  * ``-DCMAKE_INSTALL_PREFIX=<install-dir>``
  * ``-DLLVM_TABLEGEN=<path-to-host-bin>/llvm-tblgen``
  * ``-DCLANG_TABLEGEN=<path-to-host-bin>/clang-tblgen``
  * ``-DLLVM_DEFAULT_TARGET_TRIPLE=arm-linux-gnueabihf``
  * ``-DLLVM_TARGET_ARCH=ARM``
  * ``-DLLVM_TARGETS_TO_BUILD=ARM``
 
+Note: ``CMAKE_CROSSCOMPILING`` is always set automatically when ``CMAKE_SYSTEM_NAME`` is set. Don't put ``-DCMAKE_CROSSCOMPILING=TRUE`` in your options.
+
 If you're compiling with GCC, you can use architecture options for your target,
 and the compiler driver will detect everything that it needs: