Handle negative input in inlined Math.round on Intel CPUs.

R=jkummerow@chromium.org
BUG=v8:2451

Review URL: https://chromiumcodereview.appspot.com/12342037

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13764 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Miscellaneous profile-driven Isolate plumbing.

While doing this, it became clear that quite a few functions should not be
static and should better live in various classes as instance methods, but I'll
leave this for a later CL.

BUG=v8:2487

Review URL: https://codereview.chromium.org/12314152

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13765 ce2b1a6d-e550-0410-aec6-3dcde31c8c00