runtime: allow fake time writes to both stdout and stderr

In the previous sandbox implementation we read all sandboxed output
from standard output, and so all fake time writes were made to
standard output. Now we have a more sophisticated sandbox server
(see golang.org/x/playground/sandbox) that is capable of recording
both standard output and standard error, so allow fake time writes to
go to either file descriptor.

Change-Id: I79737deb06fd8e0f28910f21f41bd3dc1726781e
Reviewed-on: https://go-review.googlesource.com/2713Reviewed-by: Minux Ma <minux@golang.org>

src/runtime/sys_nacl_amd64p32.s

@@ -94,13 +94,13 @@ playback:
 	MOVL n+8(FP), DX
 	BSWAPL DX
 	MOVL DX, 12(SP)
-	MOVL $1, DI // standard output
+	MOVL fd+0(FP), DI
 	MOVL SP, SI
 	MOVL $16, DX
 	NACL_SYSCALL(SYS_write)
 
 	// Write actual data.
-	MOVL $1, DI // standard output
+	MOVL fd+0(FP), DI
 	MOVL p+4(FP), SI
 	MOVL n+8(FP), DX
 	NACL_SYSCALL(SYS_write)