MIPS: Turn ToNumberStub into a PlatformCodeStub again.

Port 724a15e8bf3fb8697a950c8e0f96158c8e56ec71
Port r25187

The HydrogenCodeStub is too expensive and there's no easy way to reduce
this cost, so turning it into a PlatformCodeStub solves that problem
until we can use TurboFan for code stubs.

TEST=mjsunit
BUG=
R=dusan.milosavljevic@imgtec.com

Review URL: https://codereview.chromium.org/707863002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25202 ce2b1a6d-e550-0410-aec6-3dcde31c8c00