Change the calling conventions for unboxed tuples slightly

Change the calling conventions for unboxed tuples slightly
When returning an unboxed tuple with a single non-void component, we
now use the same calling convention as for returning a value of the
same type as that component.  This means that the return convention
for IO now doesn't vary depending on the platform, which make some
parts of the RTS simpler, and fixes a problem I was having with making
the FFI work in unregisterised GHCi (the byte-code compiler makes
some assumptions about calling conventions to keep things simple).