runtime: fix tiny memory leak

The m->cret word holds the C return value when returning
across a stack split boundary.  It was not being cleared after
use, which means that the return value (if a C function)
or else the value of AX/R0 at the time of the last stack unsplit
was being kept alive longer than necessary.  Clear it.

I think the effect here should be very small, but worth fixing
anyway.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5677092