optimize 16- and 32-bit moves

optimize 16- and 32-bit moves

function                                             old     new   delta
udhcpd_main                                         1239    1257     +18
udhcp_add_simple_option                               93      92      -1
buffer_read_le_u32                                    19      18      -1
unpack_gz_stream_with_info                           526     520      -6
dnsd_main                                           1470    1463      -7
udhcp_run_script                                    1208    1186     -22
send_ACK                                             255     229     -26
arping_main                                         1661    1623     -38
send_offer                                           470     428     -42
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(add/remove: 0/0 grow/shrink: 1/8 up/down: 18/-143)          Total: -125 bytes