Auto merge of #117654 - alistair23:alistair/riscv-docker, r=Mark-Simulacrum

ci: docker: dist-various-1: Include RISC-V C compilers

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: rust-lang/compiler-builtins#350
2: rust-lang/compiler-builtins@e4f46b9

Author: bors