trivial packing for on-stack structs

* io.c (struct io_advise_struct): 32 => 24 bytes on 64-bit
* io.c (struct io_internal_writev_struct): 24 => 16 bytes on 64-bit
* process.c (struct waitpid_arg): ditto

Slightly reduce stack pressure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e