Windows-specific iterate_over_objfiles_in_search_order

Windows-specific iterate_over_objfiles_in_search_order

This patch sets the windows target to use their own version of
the iterate_over_objfiles_in_search_order gdbarch method, in
order to make global symbol searches sensitive to the current
objfile.

gdb/ChangeLog:

        * windows-tdep.h (windows_iterate_over_objfiles_in_search_order):
        Add declaration.
        * windows-tdep.c: #include "objfiles.h".
        (windows_iterate_over_objfiles_in_search_order): New function.
        * amd64-windows-tdep.c (amd64_windows_init_abi): Set
        iterate_over_objfiles_in_search_order gdbarch method to
        windows_iterate_over_objfiles_in_search_order.
        * i386-cygwin-tdep.c (i386_cygwin_init_abi): Likewise.