rustc: Print a smaller hash on -v

The long hash just takes up space and you can discover the main hash through the
`rustc --version verbose` command.
You should pass a length to `--short`, otherwise it will use the `core.abbrev` configuration value. The classic value is 7, but our repo already has a few commits that require 8 and one that requires 9 to be unique (at least, within my clone of the repo). I would therefore recommend going with 9, because it would be preferable for everyone to use the same hashes for the same commits).

```diff
+    CFG_SHORT_VER_HASH = $(shell git --git-dir='$(CFG_GIT_DIR)' rev-parse --short=9 HEAD)
```