scons: Detect and remove broken git hook symlinks.

scons: Detect and remove broken git hook symlinks.

When installing a git hook, it's possible for the hook to not "exist" if it's
actually a symlink which points to a file that doesn't exist. Trying to create
a new symlink in its place without first removing the old one causes a build
failure in these cases.

If the hook doesn't "exist" but is still a link, that means it's actually a
broken link and should be deleted by the hook installation function before any
new symlink is created.

Change-Id: I59aa51feb5bd74ca33e51e89cde2ceabeb41bd76
Reviewed-on: https://gem5-review.googlesource.com/2540
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>