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