vm_trace.c (rb_tp_t): pack 56 => 48 bytes on 64-bit

48 bytes is a common malloc size class on x86-64 machines which
require 16-byte alignment.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-10-03

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e