Add exception for avx2 contracts

Signed-off-by: Jake Massimo <jakemas@amazon.com>

Author: jakemas

scripts/check-contracts

@@ -46,7 +46,7 @@ def is_exception(funcname):
     if funcname == 'poly_permute_bitrev_to_custom':
         return True
 
-    if funcname.endswith("_native") or funcname.endswith("_asm"):
+    if funcname.endswith("_native") or funcname.endswith("_asm") or funcname.endswith("_avx2"):
         # CBMC proofs are axiomatized against contracts of the backends
         return True