gcc: sort the patches before they are hashed

$(wildcard ...) in make doesn't sort the files, so the order of the
hashed files is not predictable. Therefore, the ccache hash could
change from one build to another. We don't want that, so sort the
files explicitly.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>