Disable loader_attic by default on VMS

The reason is that it currently doesn't build properly, due to the of
pvkfmt.c, causing multiply defined symbols since libcrypto exports
them as well.  At the same time, it can't do without that source file,
or it won't have access to certain internal symbols from there.
Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15320)

Configurations/10-main.conf

@@ -1857,7 +1857,7 @@ my %targets = (
         asflags          => sub { vms_info()->{asflags} },
         perlasm_scheme   => sub { vms_info()->{perlasm_scheme} },
 
-        disable          => add('pinshared'),
+        disable          => add('pinshared', 'loadereng'),
 
     },