Fix issue with how symrec's are handled when an expression is copied: old code

Fix issue with how symrec's are handled when an expression is copied: old code
just assigned the pointer: but the symrec is deleted (if it's not in the symbol
table) when the expr is deleted.  Thus, we need to create a copy of the symrec
instead of just reusing the same value if it's going to be deleted later.  This
trickles down to objfmt to copy the objfmt-local data.

svn path=/trunk/yasm/; revision=467