elf: fix object::lookup_addr to lookup correct symbol

Fix object::lookup_addr to lookup correct symbol.
It should returns the nearest symbol which is s_addr < addr, but it
compares opposite way.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>