system/memory: use ldn_he_p/stn_he_p

Using direct pointer dereferencing can allow for unaligned accesses,
which was seen during execution with sanitizers enabled.

Cc: qemu-stable@nongnu.org
Reviewed-by: Chris Rauer <crauer@google.com>
Reviewed-by: Peter Foley <pefoley@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-ID: <20231116163633.276671-1-venture@google.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>