Allow specifying `cfg` expressions for `lib` targets

Hi! It looks like it's not possible to specify architecture-specific fields for a lib target. I tried adding a [target.'cfg(any(target_os="android", target_os="ios"))'.lib] section to Cargo.toml, as in mozilla/toodle#10 (comment), but Cargo warns that it's an unused manifest key.

In case it helps, the crate in question, toodle, is a library that's cross-compiled for iOS and Android, and also imported as a regular Rust crate by another crate (toodlext). crate-type = ["staticlib", "cdylib"] seems to be necessary for cross-compiling (cargo lipo fails without it), but leaving it under lib breaks toodlext, with cargo build complaining that it can't find the toodle crate (kitcambridge/sync-storage-prototype#1).

/cc @fluffyemily

[epage]

The thing I'm wondering is if it is best to handle this in a single package or to have different packages that exhibit the behavior needed for each of those platforms.