runtime: use C for readgstatus, goroutine status values

When this code was written, there was no way for Go to
reuse the C function and enum values. Now there is.

LGTM=bradfitz
R=rlh, bradfitz
CC=dvyukov, golang-codereviews, iant, khr, r
https://codereview.appspot.com/139150045