Fix gdb.texinfo for old makeinfo (again)
    
    With old makeinfo (version 4.13) the changes introduced in
    
    commit ba643918cf869fa0d064d733f69b453b6fe642ea
    Author:     Sergio Durigan Junior <sergiodj@redhat.com>
    
        Install and generate docs for gdb-add-index
    
    fail to build with
    
    gdb/doc/gdb.texinfo:2498: warning: `.' or `,' must follow @xref, not `@'.
    gdb/doc/gdb.texinfo:2517: warning: `.' or `,' must follow @xref, not `@'.
    gdb/doc/gdb.texinfo:43443: Node `gdb-add-index man' requires a sectioning command (e.g., @unnumberedsubsec).
    gdb/doc/gdb.texinfo:43443: `gdb-add-index man' has no Up field (perhaps incorrect sectioning?).
    gdb/doc/gdb.texinfo:43350: Next field of node `gdbinit man' not pointed to (perhaps incorrect sectioning?).
    gdb/doc/gdb.texinfo:43443: This node (gdb-add-index man) has the bad Prev.
    
    This patch fixes the warnings too.
    
    gdb/doc/ChangeLog:
    
    	* gdb.texinfo (set cwd): Add period.
    	(gdb-add-index man): Move anchor.
    
    (cherry picked from commit dbfa452355153093d4d6841515f9891c9572113f)