Implement socketcall and rawsocketcall in addition to seek (already
introduced in CL 100076) to support compiling with gccgo on linux/386

Change-Id: Iaa51aa159a4743ae0e394f541ff196e2d28aa27f
Reviewed-on: https://go-review.googlesource.com/117697
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>