The compiler-builtins for RISC-V are missing some key functions, such as
__bswapsi2 [1].
We can't just pull in the LLVM compiler-rt builtins as the rust-lang/rust
distribution container doesn't have a C compiler [2].
This patch adds RISC-V C compilers to the CI Dockerfile as the first
step towards enabling LLVM compiler-rt builtins for RISC-V Rust.
1: https://github.com/rust-lang/compiler-builtins/issues/350
2: e4f46b91ca
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>