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