Add 'LLVM_DEFAULT_TARGET_TRIPLE' to the documented list of CMake vari…

…ables

I always forget this variable, and the distinction between
'LLVM_TARGET_ARCH', and this variable is only documented on the 'cross
compiling' page, which I never spot and always end up searching through
the codebase for.

This patch adds the variable to the Sphinx CMake documentation so that
it is easily findable in the official LLVM docs.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D96014

llvm/docs/CMake.rst

@@ -366,6 +366,12 @@ enabled sub-projects. Nearly all of these variable names begin with
   $CMAKE_INSTALL_PREFIX/Toolchains containing an xctoolchain directory which can
   be used to override the default system tools.
 
+**LLVM_DEFAULT_TARGET_TRIPLE**:STRING
+  LLVM target to use for code generation when no target is explicitly specified.
+  It defaults to "host", meaning that it shall pick the architecture
+  of the machine where LLVM is being built. If you are building a cross-compiler,
+  set it to the target triple of your desired architecture.
+
 **LLVM_DOXYGEN_QCH_FILENAME**:STRING
   The filename of the Qt Compressed Help file that will be generated when
   ``-DLLVM_ENABLE_DOXYGEN=ON`` and