MIPS: Improve code generation for the HRandom instruction.

Port r16684 (e0f70aa)

Original commit message:
Since the per-context random number generator is now
properly seeded upon context creation, we do not need
to check for lazy-initialization anymore, and so we
can implement the HRandom instruction w/o having to
call into the C function (which means we don't need
to MarkAsCall anymore).

TEST=cctest/test-random
BUG=
R=gergely@homejinni.com

Review URL: https://codereview.chromium.org/23851028

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16695 ce2b1a6d-e550-0410-aec6-3dcde31c8c00