unix: implement pipe() on linux/mips

Change the Pipe() function to use the pipe() syscall (which has a unique
calling convention on linux/mips) instead of using pipe2(). This allows
it to work on kernels <2.6.27 when pipe2() was introduced.

Change-Id: I088886e66b594232869d8b88c6e1a630c76188a3
Reviewed-on: https://go-review.googlesource.com/126176
Reviewed-by: Vladimir Stefanovic <vladimir.stefanovic@mips.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux_mipsx.go

@@ -124,14 +124,13 @@ func Pipe2(p []int, flags int) (err error) {
 	return
 }
 
+//sysnb pipe() (p1 int, p2 int, err error)
+
 func Pipe(p []int) (err error) {
 	if len(p) != 2 {
 		return EINVAL
 	}
-	var pp [2]_C_int
-	err = pipe2(&pp, 0)
-	p[0] = int(pp[0])
-	p[1] = int(pp[1])
+	p[0], p[1], err = pipe()
 	return
 }