Alpha: define syscall_get_arch()

Since Alpha supports syscall audit it now needs to have a syscall.h
which implements syscall_get_arch() rather than hard coding this value
into audit_syscall_entry().

Based-on-patch-by: Richard Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: linux-alpha@vger.kernel.org