Toolchain names should be given numeric prefixes for clear ordering

[rickeylev]

When Bazel expands wildcards in e.g. register_toolchain("//foo/..."), it sorts the found targets lexicographical. Because the order toolchains are checked matters (the first matching wins), it's best practice to give toolchains a zero-padded numeric prefix to ensure this subtle detail is explicit and that their names can be load-bearing (generally the sort order of target names doesn't matter).

So, the toolchain() targets in cc_toolchain/BUILD.bazel should each be given a zero-padded numeric prefix.