Bootstrap: Check validity of `--target` and `--host` triples before starting a build Resolves #122128 As described in the issue, validating the `target` and `host` triples would save a lot of time before actually starting a build. This would also check for custom targets by looking for a valid JSON spec if the specified target does not exist in the [supported](https://github.com/rust-lang/rust/blob/42825768b103c28b10ce0407749acb21d32abeec/compiler/rustc_target/src/spec/mod.rs#L1401-L1689) list of targets.