syscall: prepare for 64-bit ints

This CL fixes code that incorrectly assumes that int is 32 bits wide.
Specifically, the socketpair system call expects a pointer to a pair
of int32s, not a pair of ints. Fix this inside the wrappers without
changing the APIs.

Update #2188.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6552063