Rollup merge of #123530 - 9names:build-rv32im, r=Mark-Simulacrum

Enable building tier2 target riscv32im-unknown-none-elf

riscv32im-unknown-none-elf was promoted to tier2 in
rust-lang/rust#117874
but it has not yet been added to the list of build targets.

By adding riscv32im-unknown-none-elf to the list of build targets, this PR enables end-users to install this target via rustup.