Provide EC functions that are not curve type specific

Some EC functions exist in *_GFp and *_GF2m forms, in spite of the
implementations between the two curve types being identical. This
commit provides equivalent generic functions with the *_GFp and *_GF2m
forms just calling the generic functions.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6815)