Fix arch.mli (#1993)

add symbol to archmli

Author: TheNumbat

backend/amd64/arch.mli

@@ -20,6 +20,7 @@ val popcnt_support : bool ref
 val prefetchw_support : bool ref
 val prefetchwt1_support : bool ref
 val trap_notes : bool ref
+val clmul_support : bool ref
 val sse3_support : bool ref
 val ssse3_support : bool ref
 val sse41_support : bool ref