Int64#/Word64# are defined on all platforms, but some of the operations are only available on 32-bit

It's incredibly frustrating that Int64#/Word64# are defined on all platforms (see #17375 (closed)), but the GHC.IntWord64 module, which defines useful operations like intToInt64# :: Int# -> Int64# (which can't be implemented without unsafeCoerce# on 64-bit) only on 32-bit.

I think we should be providing same operations on all platforms. On 64-bit some of these operations could just be renaming of others, some others would just be no-op.