VMS build file template: assign 'arch' to local symbol table

Since the local symbol table is looked up before the global symbol
table, 'arch' assigned in the local symbol table of the DCL where MMS
is called would be seen before the 'arch' defined in descrip.mms.
Assigning it to the local symbol table in descrip.mms removes that
issue.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1853)