Remove RISCV from LLVM_ALL_TARGETS in CMakeLists.txt

It was mistakenly added to that list in D23560 (committed in rL285712). RISCV 
is an experimental backend and should never have been in that list, I 
mistakenly interpreted LLVM_ALL_TARGETS as a list of all targets rather than 
targets to build by default. Unfortunately, because of this the RISCV backend 
has been building by default when it shouldn't be.

This commet adds a description comment, which should help to avoid such 
mistakes in the future.

See my message to llvm-dev for more information and analysis 
<http://lists.llvm.org/pipermail/llvm-dev/2017-August/116347.html>.

Differential Revision: https://reviews.llvm.org/D36538

llvm-svn: 310796

Author: asb

llvm/CMakeLists.txt

@@ -309,6 +309,7 @@ set(LLVM_CMAKE_PATH ${LLVM_MAIN_SRC_DIR}/cmake/modules)
 set(LLVM_EXAMPLES_BINARY_DIR ${LLVM_BINARY_DIR}/examples)
 set(LLVM_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/include)
 
+# List of all targets to be built by default:
 set(LLVM_ALL_TARGETS
   AArch64
   AMDGPU
@@ -320,7 +321,6 @@ set(LLVM_ALL_TARGETS
   MSP430
   NVPTX
   PowerPC
-  RISCV
   Sparc
   SystemZ
   X86