audit: __audit_syscall_entry: ignore arch arg and call syscall_get_arch() directly

Since every arch should have syscall_get_arch() defined, stop using the
function argument and just collect this ourselves.  We do not drop the
argument as fixing some code paths (in assembly) to not pass this first
argument is non-trivial.  The argument will be dropped when that is
fixed.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>