rb_econv_t: reduce to 184 bytes from 200 on 64-bit

* transcode.c (rb_econv_t): reduce to 184 bytes from 200 on 64-bit

This allows rb_econv_t to fit inside of three cache lines on x86-64.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46885 b2dd03c8-39d4-4d8f-98ff-823fe69b080e