[flang] Changes to map variables in link clause of declare target (#83643)

As per the OpenMP standard, "If a variable appears in a link clause on a
declare target directive that does not have a device_type clause with
the nohost device-type-description then it is treated as if it had
appeared in a map clause with a map-type of tofrom" is an implicit
mapping rule. Before this change, such variables were mapped as to by
default.