Keep the copied files as writable

Mac's strip tool doesn't work if the file is read-only. Some of the
libraries we copy might be read-only (like those installed by
HomeBrew) and that would cause stripping to fail.

Change-Id: Ie8caa853a26794ec97ec84354d9f7bb142cce993
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>