runtime: allow preemption in fast syscall return

Let a fast syscall return be a preemption point.  This helps with
tight loops that make system calls, as in BenchmarkSyscallExcessWork.

Change-Id: I9c723398cf1678afb70ca8281e9aa0339b98b8dc
Reviewed-on: https://go-review.googlesource.com/94895
Reviewed-by: Than McIntosh <thanm@google.com>
1 file changed