Configurations/unix-Makefile.tmpl: make cleanup kinder

Configurations/unix-Makefile.tmpl: make cleanup kinder

The removal of certain types of files we structured like this:

    -$(RM) `find . {{options}} -print`

This isn't very kind for shells with limited command line lengths
(even when that limit is generous, in our case), so we rewrite those
like this:

    -find . {{options}} -exec $(RM) {} \;

Fixes #12938

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/12939)