Add missing documentation for running tests with GCC backend

Author: GuillaumeGomez

src/tests/codegen-backend-tests/cg_gcc.md

@@ -1,3 +1,20 @@
 # GCC codegen backend tests
 
-TODO: please add some more information to this page.
+To test the GCC codegen backend, you need to add `"gcc"` into the `rust.codegen-backends`
+setting in `bootstrap.toml`:
+
+```toml
+rust.codegen-backends = ["llvm", "gcc"]
+```
+
+If you don't want to build `gcc` yourself, you also need to set:
+
+```toml
+gcc.download-ci-gcc = true
+```
+
+Then when running tests, add the `--test-codegen-backend gcc` option. For example:
+
+```bash
+./x.py test tests/ui --test-codegen-backend gcc
+```