Add EWOULDBLOCK to sycall_nacl.go.

R=rsc
CC=go-dev, golang-dev
https://golang.org/cl/1006045

src/pkg/syscall/syscall_nacl.go

@@ -108,6 +108,10 @@ func Sleep(ns int64) (errno int) {
 //	SYS_TLS_*
 //	SYS_SCHED_YIELD
 
+// #define'd in NaCl but not picked up by mkerrors_nacl.sh.
+
+const EWOULDBLOCK = EAGAIN
+
 // Not implemented in NaCl but needed to compile other packages.
 
 const (