cmd/gc: prepare for 64-bit ints

cmd/gc: prepare for 64-bit ints

This CL makes the compiler understand that the type of
the len or cap of a map, slice, or string is 'int', not 'int32'.
It does not change the meaning of int, but it should make
the eventual change of the meaning of int in 6g a bit smoother.

Update #2188.

R=ken, dave, remyoudompheng
CC=golang-dev
https://golang.org/cl/6542059