runtime: mark usleep as NOSPLIT on Plan 9
runtime: mark usleep as NOSPLIT on Plan 9

Usleep is called from lockextra, also marked NOSPLIT.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9258043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang