Add 'LLVM_DEFAULT_TARGET_TRIPLE' to the documented list of CMake variables

[dylanmckay]

Summary

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.

[probinson]

Seems reasonable, provided you correct the language to be about building a cross-compiler rather than actually cross-compiling.

llvm/docs/CMake.rst

352

If you are building a cross-compiler, ...

[dylanmckay]

llvm/docs/CMake.rst

352

Very good point, I will correct.

llvm/docs/CMake.rst

**LLVM_BUILD_32_BITS**:BOOL
   available only on some 64-bit Unix systems. Defaults to OFF.
 
 **LLVM_TARGET_ARCH**:STRING
   LLVM target to use for native code generation. This is required for JIT
   generation. It defaults to "host", meaning that it shall pick the architecture
   of the machine where LLVM is being built. If you are cross-compiling, set it
   to the target architecture name.
 
+**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 cross-compiling, set it
+  to the target triple of your desired architecture.
+
 **LLVM_TABLEGEN**:STRING
   Full path to a native TableGen executable (usually named ``llvm-tblgen``). This is
   intended for cross-compiling: if the user sets this variable, no native
   TableGen will be created.
 
 **LLVM_LIT_ARGS**:STRING
   Arguments given to lit.  ``make check`` and ``make clang-test`` are affected.
   By default, ``'-sv --no-progress-bar'`` on Visual C++ and Xcode, ``'-sv'`` on