external-sourcery-toolchain: only copy existing /usr/ subdirs

It seems there are versions of the toolchain which provide empty directories,
which is a problem for anyone storing their toolchain in source control. So
ensure we only ever copy the paths that exist. Previously, we had left it as
is to make sure it *would* break if a path we require is missing, but
admittedly not all of these paths are critical, and if something is that badly
broken in the toolchain, other things are likely to break elsewhere anyway.

Signed-off-by: Christopher Larson <kergoth@gmail.com>