VMS: build ia64 assembler files if 'ias' is available

Avoid using crypto/modes/ghash-ia64.s, as it uses features that are
explicitely prohibited on VMS.
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5357)

Configurations/10-main.conf

@@ -1925,9 +1925,13 @@ my %targets = (
         pointer_size     => "64",
     },
     "vms-ia64" => {
-        inherit_from     => [ "vms-generic" ],
+        inherit_from     => [ "vms-generic",
+                              sub { vms_info()->{as}
+                                        ? asm("ia64_asm")->() : () } ],
         bn_ops           => "SIXTY_FOUR_BIT RC4_INT",
         pointer_size     => "",
+
+        modes_asm_src    => "", # Because ghash-ia64.s doesn't work on VMS
     },
     "vms-ia64-p32" => {
         inherit_from     => [ "vms-ia64" ],