syscall: Permit non-blocking syscalls.

syscall: Permit non-blocking syscalls.

Permit system calls to be designated as non-blocking, meaning
that we simply call them without involving the scheduler.

This change by itself is mostly performance neutral.  In
combination with a following change to the net package there
is a performance advantage.

R=rsc, dfc, r2, iant2, rsc1
CC=golang-dev
https://golang.org/cl/4278055