Add support for armv7-unknown-linux-{gnu,musl}eabi

The ARMv7 soft-float targets were [new in rustc 1.38](https://github.com/rust-lang/rust/commit/2b0f4483d2f16e6c51a276665bff750fc40c9da9). It would be good to have images for `armv7-unknown-linux-gnueabi` and `armv7-unknown-linux-musleabi`.