syscalls: implement getpeername()

getpeername() is required by iperf in client mode

without it iperf crashes as following:

[/]% run /tools/iperf.so -c 10.0.0.144
Failed looking up symbol getpeername

[backtrace]
0x00000000003326a2 <elf::object::resolve_pltgot(unsigned int)+82>
0x0000000000369b42 <__elf_resolve_pltgot+46>
0x0000100004207535 <Client::Client(thread_Settings*)+301>
0x0000100004208b44 <client_spawn+50>
0x000010000421323a <thread_stop+272>
0x0000000000412ca5 <sync+69>
0x00000000003b2eea <thread_main_c+26>
0x000000000036ae95 <thread_main+7>

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>