[Docs] Mention LLVM_EXPERIMENTAL_TARGETS_TO_BUILD variable in CMake.rst

Beginners might not aware of this variable and wanted to try a new experimental target.

Although this variable mention in Writing a Backend Documentation. But it becomes easy to search when listed in cmake.rst doc where most variables are listed.

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D100729