[docs] Fix cross-compile instructions

[loladiro]

Summary

Setting the CMAKE_CROSSCOMPILING variable directly does not appear to work.
Instead, CMake is setting this variable based on the presence
or absence of the CMAKE_SYSTEM_NAME variable (in fact, if that variable is
not present, CMAKE_CROSSCOMPILING gets reset to false [1]). Lastly,
building an external tablegen is not necessary. LLVM is capable of
building a native tablegen as needed.

[1] https://github.com/Kitware/CMake/blob/eae3765b67b653d3f00afa44a60719a387262af8/Modules/CMakeDetermineSystem.cmake#L126

docs/HowToCrossCompileLLVM.rst

 Configuring CMake
 -----------------
 
 For more information on how to configure CMake for LLVM/Clang,
 see :doc:`CMake`.
 
 The CMake options you need to add are:
 
- * ``-DCMAKE_CROSSCOMPILING=True``
+ * ``-DCMAKE_SYSTEM_NAME=Linux`` (this is needed even if the build system is already Linux -
+   the presence of this flag causes CMake to enter cross-compilation mode)
  * ``-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``
+ * ``-DCMAKE_C_COMPILER`` and ``-DCMAKE_CXX_COMPILER`` if your cross capable compiler is not
+   the same as your default compiler.
 
 If you're compiling with GCC, you can use architecture options for your target,
 and the compiler driver will detect everything that it needs:
 
  * ``-DCMAKE_CXX_FLAGS='-march=armv7-a -mcpu=cortex-a9 -mfloat-abi=hard'``
 
 However, if you're using Clang, the driver might not be up-to-date with your
 specific Linux distribution, version or GCC layout, so you'll need to fudge.
 
 In addition to the ones above, you'll also need:
 
  * ``'-target arm-linux-gnueabihf'`` or whatever is the triple of your cross GCC.
  * ``'--sysroot=/usr/arm-linux-gnueabihf'``, ``'--sysroot=/opt/gcc/arm-linux-gnueabihf'``
    or whatever is the location of your GCC's sysroot (where /lib, /bin etc are).
  * Appropriate use of ``-I`` and ``-L``, depending on how the cross GCC is installed,
    and where are the libraries and headers.
 
-The TableGen options are required to compile it with the host compiler,
-so you'll need to compile LLVM (or at least ``llvm-tblgen``) to your host
-platform before you start. The CXX flags define the target, cpu (which in this case
+The CXX flags define the target, cpu (which in this case
 defaults to ``fpu=VFP3`` with NEON), and forcing the hard-float ABI. If you're
 using Clang as a cross-compiler, you will *also* have to set ``--sysroot``
 to make sure it picks the correct linker.
 
 When using Clang, it's important that you choose the triple to be *identical*
 to the GCC triple and the sysroot. This will make it easier for Clang to
 find the correct tools and include headers. But that won't mean all headers and
 libraries will be found. You'll still need to use ``-I`` and ``-L`` to locate