[Docs][WritingAnLLVMBackend] Makefiles are deprecated

Summary: * docs/WritingAnLLVMBackend.rst: Makefiles are no longer used. The users should use CMakeLists.txt. In order to add the target, the TARGETS_TO_BUILD is replaced with LLVM_ALL_TARGETS.

Reviewers: gribozavr, void, beanz

Subscribers: llvm-commits

Patch By: Visoiu Mistrih Francis (thegameg)

Differential Revision: http://reviews.llvm.org/D20700

llvm-svn: 270921