mips: fix syscall_pipe

Previously the fd parameter was ignored and so the result would not get
populated. Now it passes the fd pointer to the inline assembly so that
the results can be observed.

lib/std/os/linux/mips.zig

@@ -31,7 +31,8 @@ pub fn syscall_pipe(fd: *[2]i32) usize {
         \\ sw $3, 4($4)
         \\ 2:
         : [ret] "={$2}" (-> usize)
-        : [number] "{$2}" (@enumToInt(SYS.pipe))
+        : [number] "{$2}" (@enumToInt(SYS.pipe)),
+          [fd] "{$4}" (fd)
         : "memory", "cc", "$7"
     );
 }