Rollup merge of #147633 - GuillaumeGomez:bypass-ignore-backends, r=kobzol

Add new `--bypass-ignore-backends` option

Fixes rust-lang/rust#147063.

It adds a new option to `bootstrap` to allow to ignore `//@ ignore-backends` statements in tests.

cc ```@jieyouxu``` ```@antoyo```
r? ```@Kobzol```

Author: matthiaskrgr

src/tests/directives.md

@@ -207,7 +207,9 @@ 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.
+- `ignore-backends` — ignores the listed backends, separated by whitespace characters. Please note
+  that this directive can be overriden with the `--bypass-ignore-backends=[BACKEND]` command line
+  flag. 
 - `needs-backends` — only runs the test if current codegen backend is listed.
 
 The following directives will check LLVM support: