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.
    
    Reviewed-on: https://go-review.googlesource.com/94895

From-SVN: r257848

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-459a8a94e04a19bde7173ef7cf2db369c2e62e2d
+c6e0970f75508e209a10a7db5164d6ea3f9b28bf
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -2794,6 +2794,13 @@ func exitsyscall(dummy int32) {
 		exitsyscallclear(_g_)
 		_g_.m.locks--
 		_g_.throwsplit = false
+
+		// Check preemption, since unlike gc we don't check on
+		// every call.
+		if getg().preempt {
+			checkPreempt()
+		}
+
 		return
 	}