runtime: fix usleep by correctly setting nanoseconds parameter for pselect6

Fixes #21518

Change-Id: Idd67e3f0410d0ce991b34dcc0c8f15e0d5c529c9
Reviewed-on: https://go-review.googlesource.com/56850Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_linux_386.s

@@ -100,7 +100,7 @@ TEXT runtime·usleep(SB),NOSPLIT,$8
 	MOVL	AX, 0(SP)
 	MOVL	$1000, AX	// usec to nsec
 	MULL	DX
-	MOVL	DX, 4(SP)
+	MOVL	AX, 4(SP)
 
 	// pselect6(0, 0, 0, 0, &ts, 0)
 	MOVL	$308, AX