Rollup merge of #144125 - GuillaumeGomez:new-annotations, r=Kobzol

Add new `ignore-backends` and `needs-backends` tests annotations

Part of rust-lang/compiler-team#891.

Next step will be to add these annotations in the files where either the output is different based on the codegen (like `asm` tests) or that are known to fail in the GCC backend.

cc `@oli-obk` `@antoyo`
r? `@Kobzol`

Author: GuillaumeGomez

src/tests/directives.md

@@ -205,6 +205,8 @@ 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.
+- `needs-backends` — only runs the test if current codegen backend is listed.
 
 The following directives will check LLVM support: