[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.