misc/cgo: prepare for 64-bit ints

misc/cgo: prepare for 64-bit ints

In a few places, the existing cgo tests assume that a
Go int is the same as a C int. Making int 64 bits wide
on 64-bit platforms violates this assumption.
Change that code to assume that Go int32 and C int
are the same instead. That's still not great, but it's better,
and I am unaware of any systems we run on where it is not true.

Update #2188.

R=iant, r
CC=golang-dev
https://golang.org/cl/6552064