Allow specifying exec and target compatibility constraints

[illicitonion]

Extra target triples may only be appropriate for specific platforms (e.g. if you register a toolchain with the exec_triple x86_64-unknown-linux-gnu with an extra target triple of x86_64-unknown-linux-musl, you may want differing target_compatible_with values for each of those two target triples.

This allows for specifying explicit exec_compatible_with lists per toolchain (which was already supported by rust_toolchain_repository just not proxied by rust_repository_set), and explicit target_compatible_with lists, both for the default triple and for extra ones.

I'm not 100% sure on our pre-existing assumption that every toolchain supports compiling targeting its own exec platform, i.e. the '[exec_triple]' part of

rules_rust/rust/repositories.bzl

Line 845 in 937e633

for target_triple in [exec_triple] + extra_target_triples:

but it does generally hold, and supplying a way of removing that default assumption is definitely a separate PR for another day if it becomes relevant to anyone.

[UebelAndre]

I think this seems fine but it would be nice to see an example of it somewhere to both inform users of how they can be more descriptive with their toolchains while also acting as a regression test.

[illicitonion]

Thanks! I'll try to get the relevant pieces open sourced so we could have a working example :) Unfortunately toolchains are a bit fiddly in general, but hopefully this shouldn't be too hard to put together :)