We use fstp(0) already in other places. ffree does not pop the stack,
but only marks a FP register as empty.

BUG=v8:2065
Review URL: https://chromiumcodereview.appspot.com/9960066

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11255 ce2b1a6d-e550-0410-aec6-3dcde31c8c00