Split Makefile clean recipe for document sets into individual lines.
This is needed for less capable platforms with limits on the size of
command line argument lists.
Fixes #14732
CLA: The author has the permission to grant the OpenSSL Team the right to use this change.
Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com> Reviewed-by: Richard Levitte <levitte@openssl.org> Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14735)