MIPS: Improve code generation for the HRandom instruction.

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=

[kisg, 2013-09-12 21:06:47]

LGTM

[palfia, 2013-09-12 21:18:14]

Committed as r16695.