Add new ignore-backends tests annotations

Author: GuillaumeGomez

src/tests/directives.md

@@ -205,6 +205,7 @@ settings:
   on `wasm32-unknown-unknown` target because the target does not support the
   `proc-macro` crate type.
 - `needs-target-std` — ignores if target platform does not have std support.
+- `ignore-backends` — ignores the listed backends, separated by whitespace characters.
 
 The following directives will check LLVM support: