vm_eval.c (rb_eval_cmd): use pre-defined idCall

No need to use rb_intern, here.  Reduces size slightly on x86-64:

$ ~/linux/scripts/bloat-o-meter ruby.before ruby.after
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-31 (-31)
function                                     old     new   delta
rb_eval_cmd                                  813     782     -31

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48418 b2dd03c8-39d4-4d8f-98ff-823fe69b080e