rb_iseq_t: reduce to (280 bytes from 288 bytes) on 64-bit

iseq_size and line_info_size may be 32-bit.

4GB instruction sequences should be big enough for anyone.
Other existing line info counters are 32-bit, and nobody
should need all that.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e