arch/x86: add syscall_get_arch to syscall.h

Add syscall_get_arch() to export the current AUDIT_ARCH_* based on system call
entry path.

Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Reviewed-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Eric Paris <eparis@redhat.com>

v18: - update comment about x32 tasks
     - rebase to v3.4-rc2
v17: rebase and reviewed-by
v14: rebase/nochanges
v13: rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc