Add new `thumbv7neon_linux_androideabi` and `thumbv7neon-unknown-linux-gnueabihf` targets

[briansmith]

Originally Rust's armv7- targets hard-coded NEON support. However, that was changed in rust-lang/rust#47765.

rust-lang/rust#49897 tracks adding new thumbv7neon- targets that act like the old targets. See also the PR rust-lang/rust#56947 that adds them.

These targets are very useful since a lot of the time we know that NEON is going to be available on the target. The Rust issue above notes "Firefox opts not to support ARMv7 devices that don't have it," for example. I've also had people ask for versions of my software that assume NEON is available.