[Docs][WritingAnLLVMBackend] Makefiles are deprecated
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@270921 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm