Auto merge of #126641 - ferrocene:hoverbear/enable-riscv64gc-testing, r=<try>

Enable `riscv64gc-gnu` testing

Together with joshua.zivkovic@codethink.co.uk, we've been starting to explore improving the state of the `riscv64gc-unknown-linux-gnu` target. Additionally, I'm looking to add support for this platform in [Ferrocene](https://github.com/ferrocene/ferrocene) ([Related PR](ferrocene/ferrocene#618)).

Recently several PRs have landed improving the state of this target:

* #125220
* #125669
* #126355
* #126279
* #126707
* #126916
* #126917
* #127280
* #127967

The result has been that `riscv64gc-unknown-linux-gnu` now *should* pass the same CI tests that `x86_64-unknown-linux-gnu` and `aarch64-unknown-linux-gnu` do.

## Testing

> [!NOTE]
> While `riscv64gc-unknown-linux-gnu` is a [**Tier 2 with Host Tools** platform](https://doc.rust-lang.org/beta/rustc/platform-support.html) (meaning all tests may not necessarily pass) we do need to see all of the tests passing here. Indeed, the point of this PR is to get `riscv64gc-unknown-linux-gnu` into automated testing so the tests can *remain* working.

You can test out the job locally:

```sh
DEPLOY=1 ./src/ci/docker/run.sh riscv64gc-gnu
```

`DEPLOY=1` helps reproduce the CI's environment and also avoids the chance of a `llvm-c/BitReader.h` error (detailed in #85424 and #56650).

try-job: riscv64gc-gnu

Author: bors

src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile renamed to src/ci/docker/host-x86_64/riscv64gc-gnu/Dockerfile

@@ -205,6 +205,9 @@ auto:
   - image: mingw-check
     <<: *job-linux-4c
 
+  - image: riscv64gc-gnu
+    <<: *job-linux-4c
+
   - image: test-various
     <<: *job-linux-4c