Update rust_toolchain.target_json from a label to a json string

[UebelAndre]

There have been a couple of cases where the functionality added in #836 to teach rust_toolchain to handle json files which represent fine grain information about platform triples has caused confusion or a gap in information in "starlark-land" when writing rules. A common issue is that while you can use target_json, you still need to provide os and in some cases, a "most similar" target_arch but that can differ from what target_json actually represents. Instead of supporting either a json or explicitly defined values, I think everything should be written to the toolchain. For folks who still want to use a json file to represent their platform triples, I suggest a repository rule be updated or introduced to allow translating the json file into explicit values available in "starlark-land".

[UebelAndre]

cc @hlopko

[hlopko]

cc @krasimirgg