misc: Add the legacy gem5 .git/hooks to the pre-commit

The gem5 commit msg checker must be run during the 'commit-msg'
stage ergo this is explicitly set. The other hooks are only applicable
to the "commit" stage, the `default_stage` for the hooks.

To install all the hooks, you need to run the following:

```
pre-commit install -t pre-commit -t commit-msg
```

This ensures both the 'commit-msg' and 'pre-commit' hooks are installed.
If you run just `pre-commit install`, only the pre-commit hooks are
installed.

Change-Id: I4a0dcc7159ed5048baa120adf80bbf65f63c11dd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62552
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

.pre-commit-config.yaml

@@ -47,6 +47,8 @@ exclude: |
     src/python/m5/ext/.*
   )$
 
+default_stages: [commit]
+
 repos:
 - repo: https://github.com/pre-commit/pre-commit-hooks
   rev: v4.3.0
@@ -63,3 +65,18 @@ repos:
   rev: 22.6.0
   hooks:
     - id: black
+- repo: local
+  hooks:
+  - id: gem5-style-checker
+    name: gem5 style checker
+    entry: util/git-pre-commit.py
+    always_run: true
+    exclude: ".*"
+    language: system
+    description: 'The gem5 style checker hook.'
+  - id: gem5-commit-msg-checker
+    name: gem5 commit msg checker
+    entry: ext/git-commit-msg
+    language: system
+    stages: [commit-msg]
+    description: 'The gem5 commit message checker hook.'