runtime: fix int64 reconstruction in semasleep
runtime: fix int64 reconstruction in semasleep

I had this right in one of my clients, but apparently not the one I submitted from.

Fixes 386 builds.

TBR=dfc
CC=golang-codereviews
https://codereview.appspot.com/138000045

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go