Always 16-byte align |poly| elements.

Even if the vector code isn't used in hrss.c, it might call external
assembly that still requires alignment.

Change-Id: I11ceb88f96deec6b20883872030ca090506ca150
Reviewed-on: https://boringssl-review.googlesource.com/c/33584
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>

crypto/hrss/hrss.c

diff --git a/crypto/hrss/hrss.c b/crypto/hrss/hrss.c
index c059b83..b7002c1 100644
--- a/crypto/hrss/hrss.c
+++ b/crypto/hrss/hrss.c
@@ -1022,7 +1022,9 @@
     vec_t vectors[VECS_PER_POLY];
   };
 #else
-  uint16_t v[N + 3];
+  // Even if !HRSS_HAVE_VECTOR_UNIT, external assembly may be called that
+  // requires alignment.
+  alignas(16) uint16_t v[N + 3];
 #endif
 };