[RISCV] Add RISCV to LLVM_ALL_TARGETS so it s built by default

This follows the RFC <http://lists.llvm.org/pipermail/llvm-dev/2019-July/133724.html>.

Follow-on commits will add appropriate release notes changes etc.

Pushing this now and in a minimal form so there is reasonable time before 9.0
branches to resolve any issues arising from e.g. the backend being exposed on
different sanitizer setups.

The current builder for RISC-V is on the staging build-bot
<http://lab.llvm.org:8014/builders/llvm-riscv-linux>, however with the RISCV
backend being built by default it won't provide any real additional coverage.
We will shortly set up a builder that runs the test-suite in qemu-user.

llvm-svn: 366331

llvm/CMakeLists.txt

@@ -285,6 +285,7 @@ set(LLVM_ALL_TARGETS
   MSP430
   NVPTX
   PowerPC
+  RISCV
   Sparc
   SystemZ
   WebAssembly