runtime/race: update race runtime before Go 1.5 release

[minux]

@dvyukov might also want to discuss how best to align the update
of the race runtime with our release cycle.

Personally I don't mind updating the race runtime more often, once
some user visible bugs are fixed. Now that we have gomote, building
the race runtime on all supported platforms shouldn't be a problem.
(Ask bradfitz about gomote. I think at least Linux and FreeBSD are
covered.)

[dvyukov]

#8653 is related to more frequent updates