Add support for alignment_assumptions in uubsan

Cherry-pick from FreeBSD:

From 7c1bc5ffc2fa68ddc76e5ea8a3a1a6fdfeee57f0 Mon Sep 17 00:00:00 2001
From: andrew <andrew@FreeBSD.org>
Date: Tue, 28 May 2019 09:12:15 +0000
Subject: [PATCH] Teach the kernel KUBSAN runtime about alignment_assumption

This checks the alignment of a given pointer is sufficient for the
requested alignment asked for. This fixes the build with a recent
llvm/clang.

Sponsored by: DARPA, AFRL

(kamil)