Add 'LLVM_DEFAULT_TARGET_TRIPLE' to the documented list of CMake variables

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