CHROMIUM: arch/arm: add asm/syscall.h

(I will post this upstream after the 3.5 merge window)

Provide an ARM implementation for asm-generic/syscall.h.
This is a pre-requisite for CONFIG_HAVE_ARCH_TRACEHOOK and
CONFIG_HAVE_ARCH_SECCOMP_FILTER.  The latter is the forcing
function for this patch.

Change-Id: Idc5fa7b72691ec9d75418849733633df33482e53
Signed-off-by: Will Drewry <wad@chromium.org>

BUG=chromium-os:27878
TEST=compiles for arm. Need to test on a live machine.

Change-Id: I7b911b51085424aedd2beaf40683c3348b6cede1
Reviewed-on: https://gerrit.chromium.org/gerrit/21375
Reviewed-by: Will Drewry <wad@chromium.org>
Tested-by: Will Drewry <wad@chromium.org>
Signed-off-by: Sasha Levitskiy <sanek@google.com>