Improve Ring configuration for seL4

[nspin]

briansmith/ring#1937 enables Ring to be built with target_os = "none", but the resulting configuration doesn't leverage hand-optimized assembly.

Adding some kind of configuration modifier (e.g. a Cargo feature) that adds target_os = "none" to the LINUX_ABI list at https://github.com/briansmith/ring/blob/befdc87ac7cbca615ab5d68724f4355434d3a620/build.rs#L238-L251 would do the trick.