Search global symbols from the expression's block objfile first.

gdb/ChangeLog:

        * findvar.c (default_read_var_value): For LOC_UNRESOLVED symbols,
        try locating the symbol in the symbol's own objfile first, before
        extending the search to all objfiles.
        * symtab.c (lookup_symbol_aux_objfile): New function, extracted
        out of lookup_symbol_aux_symtabs.
        (lookup_symbol_aux_symtabs): Add new parameter "exclude_objfile".
        Replace extracted-out code by call to lookup_symbol_aux_objfile.
        Do not search EXCLUDE_OBJFILE.
        (lookup_static_symbol_aux): Update call to lookup_symbol_aux_symtabs.
        (lookup_symbol_global): Search for matches in the block's objfile
        first, before searching all other objfiles.