Redefine gcdInt to use gcdInteger rather than gcdInt# primop

Redefine gcdInt to use gcdInteger rather than gcdInt# primop
The gcdInt# primop uses gmp internally, even though the interface is
just Int#. Since we want to get gmp out of the rts we cannot keep
gcdInt#, however it's also a bit odd for the integer package to export
something that doesn't actually use Integer in its interface. Using
gcdInteger is still not terribly satisfactory aesthetically. However
in the short-term it works and it is no slower since gcdInteger calls
gcdInt# for the special case of two small Integers.