Fix using ZeroMemset for Serial

[masterleinad]

Fixes #5076. There is a specialization for DefaultHostExecutionSpace that we would not use for Serial when there is another host parallel execution space.

[masterleinad]

The code for the default host space is

kokkos/core/src/Kokkos_HostSpace.hpp

Lines 280 to 299 in 33d0285

	template <class DT, class... DP>
	struct ZeroMemset<typename HostSpace::execution_space, DT, DP...> {
	ZeroMemset(const typename HostSpace::execution_space& exec,
	const View<DT, DP...>& dst,
	typename View<DT, DP...>::const_value_type&) {
	// Host spaces, except for HPX, are synchronous and we need to fence for HPX
	// since we can't properly enqueue a std::memset otherwise.
	// We can't use exec.fence() directly since we don't have a full definition
	// of HostSpace here.
	hostspace_fence(exec);
	using ValueType = typename View<DT, DP...>::value_type;
	std::memset(dst.data(), 0, sizeof(ValueType) * dst.size());
	}
	ZeroMemset(const View<DT, DP...>& dst,
	typename View<DT, DP...>::const_value_type&) {
	using ValueType = typename View<DT, DP...>::value_type;
	std::memset(dst.data(), 0, sizeof(ValueType) * dst.size());
	}
	};

.

[masterleinad]

The failing jenkins test is

[ RUN      ] openmptarget.unique_token_global
4: /var/jenkins/workspace/Kokkos/core/unit_test/TestUniqueToken.hpp:151: Failure
4: Expected equality of these values:
4:   sum
4:     Which is: 9999990
4:   int64_t(N) * R
4:     Which is: 10000000

which looks unrelated.