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

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

The search order used in this patch breaks global symbol lookups
for certain symbols when copy-relocation is used.  A slightly different
search order will be implemented later.

gdb/ChangeLog:

        Revert the following patch:
        * 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.