Allow `cfg(target="..")`

Looks like right now it's impossible to match on target triplet directly in cfg expressions. Example (compile to WASM). Is it intentional?

Rust issue: #63217

Under what conditions would you need the whole triple, as opposed to individual parts of it (which could be combined with all() as needed)?

One use case was to work around target_vendor being unstable on Rust 1.32, another use-case is to specify cfg branch specifically for wasm32-unknown-unknown and being future-proof against potential future addition of new WASM targets. Either way, using a concrete target triple is more convenient, easier to read and less error-prone compared to the combined all expressions.