Support builtins for RISC-V, RV32 and RV64.
Thanks Kito, with this change plus lib/Basic/Targets/RISCV.h change in clang to return true for hasInt128Type() and disabling fPIC we can build compiler rt for RISCV. We will test it next. But we need to decide if we want to go this route or implement int128_t with int64_t.
- Add testcase.
- Remove muldi3 support for RV64 in lib/builtins/riscv/mul3.S, because compiler-rt already provide a general version for muldi3.
- Rename lib/builtins/riscv/mul3.S to lib/builtins/riscv/mulsi3.S for matching compiler-rt's naming convention.