Revert "s390: Deprecate ESA/390 support"

Revert "s390: Deprecate ESA/390 support"

The intention of -m31 -mesa and -m31 -mzarch was that they are (ABI)
compatible which is almost true except as it turns out they are not for
attribute mode(word).

After doing some archaeology and digging out an over 18 year old thread
[1,2] which is about this very attribute, I come to the conclusion to
revert this patch.  The intention by deprecating and eventually removing
ESA/390 support was to prepare for a future removal of -m31; though in
smaller steps.  Thus, instead of introducing some potential hick ups
along the route, I will revert this patch and will revisit this topic
when time for -m31 in its entirety has come---independent of
-mesa/-mzarch.

[1] https://gcc.gnu.org/pipermail/gcc-patches/2006-September/200465.html
[2] https://gcc.gnu.org/pipermail/gcc-patches/2006-October/201154.html

This reverts commit 3b1bd1fdcd241dd1e5b706b6937400d74ca43146.