bazel: Migrate to the native x86_64-unknown-none build target #14

Now that x86_64-unknown-none is a Tier 2 target (see rust-lang/rust#95705), we should be able to fetch it as a pre-built release. At the very least, this should mean we no longer need to fetch the standard library sources, build libcore and liballoc (and possibly compiler-builtins too). They should be replaced automatically by specifying x86_64-unknown-none in extra_target_triples in rust_register_toolchains.

If we're really lucky, it might even be possible to drop the entire cross-compiling toolchain for Rust. That would simplify things significantly.