internal/rosa/stage0: add riscv64 tarball

This had not yet passed all test suites because emulator is prohibitively slow.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/llvm.go

@@ -207,6 +207,8 @@ func (t Toolchain) newLLVM() (musl, compilerRT, runtimes, clang pkg.Artifact) {
 		target = "X86"
 	case "arm64":
 		target = "AArch64"
+	case "riscv64":
+		target = "RISCV"
 
 	default:
 		panic("unsupported target " + runtime.GOARCH)