[CMake][fuzzer] Add riscv64 to fuzzer supported arch list

I tried to build libFuzzer for RISC-V and succeeded. All the libFuzzer
targets were successfully built. I tested this on the small hello world code
with a few branches to check the instrumentation; all of them were covered by
libFuzzer on RISC-V arch. So I suppose it makes sense to enable libFuzzer
build for RISC-V.

Reviewed By: phosek, thetruestblue, MaskRay

Differential Revision: https://reviews.llvm.org/D147788