rts: fix top handler closure type signatures

This commit fixes the runIO/runNonIO closure type signatures in the
RTS which should be extern StgClosure. This allows us to remove an
unnecessary type cast in the C foreign desugaring logic, as well as
unneeded complications of JSFFI desugaring logic that also needs to
generate C stubs that may refer to those top handler closures.
Otherwise, we'll have to take special care to avoid generating "extern
StgClosure" declarations for them as we would for other closures, just
to avoid conflicting type signature error at stub compile time.

(cherry picked from commit c78d8f55)