Smarter build of system error text database

We stored copies of the system error texts in a fixed line size array,
which is a huge waste.  Instead, use a static memory pool and pack all
the string in there.  The wasted space at the end, if any, gives us
some leeway for longer strings than we have measured so far.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7681)