Add a target to generate local coverage reports

Add a target to generate local coverage reports

Add a target to allow developers to generate coverage reports for a
build like coveralls does.  This will let us identify rarely used
branches when doing development, so that we can optimize them.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/27839)