i686-unknown-redox is supported by stage0 now

src/bootstrap/src/core/sanity.rs

@@ -34,7 +34,6 @@ pub struct Finder {
 // Targets can be removed from this list once they are present in the stage0 compiler (usually by updating the beta compiler of the bootstrap).
 const STAGE0_MISSING_TARGETS: &[&str] = &[
     // just a dummy comment so the list doesn't get onelined
-    "i686-unknown-redox",
     "riscv64gc-unknown-redox",
 ];