rb_proc_t: reduce to 64 bytes from 72 on 64-bit

* vm_core.h (rb_proc_t): reduce to 64 bytes from 72 on 64-bit

This allows rb_proc_t to fit inside a single cache line on x86-64

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46886 b2dd03c8-39d4-4d8f-98ff-823fe69b080e