Update README for target information.

[Alexhuszagh]

Updated target information using the target-info command from cross-dev. Also added test detection for cross-dev, and fixed a bug with powerpc64le-unknown-linux-gnu using the wrong qemu architecture. I also removed targets for which we don't provide images, these are commented out below the table, so if we re-enable them they aren't gone entirely.

[Alexhuszagh]

UPDATE: I've made all the changes below.

We probably could remove some of these footnotes and remove some of the commented-out targets entirely.

This isn't due to cross, since we don't provide an image and is just handled by the host toolchain.

[1] iOS cross compilation is supported on macOS hosts.

This can just say no std component since I hope no one is using a compiler pre-2018 anymore, which will get duplicated if we ever add back x86_64-sun-solaris, which is also tier 3 now.

[3] No `std` component available as of 2017-01-10

We can comment this out currently as well until we restore the emscripten targets:

[6] libc = musl, gcc = emcc. The Docker images for these targets are currently not built automatically
due to a [compiler bug](https://github.com/rust-lang/rust/issues/85821), you will have to build them yourself for now.

So we should be able to remove [1], shift everything else down by 1, modify [3], and then comment out [6].

We'll also need to add the thumbv7* targets on #745 merges.

[Emilgardis]

missed the update! lgtm

bors r=otavio

[bors]

Build succeeded:

• conclusion