backporting gdb build failure fix to binutils 2.30

[Szabolcs Nagy]

i'd like to backport these gdb documentation patches
into the binutils 2.30 branch, so make install can
succeed even with an old texinfo:

https://sourceware.org/ml/gdb-patches/2018-01/msg00259.html
https://sourceware.org/ml/gdb-patches/2018-01/msg00266.html

i opened https://sourceware.org/bugzilla/show_bug.cgi?id=23156

commit dbfa452355153093d4d6841515f9891c9572113f
Author:     Szabolcs Nagy <szabolcs.nagy@arm.com>
AuthorDate: 2018-01-15 10:44:12 +0000
Commit:     Szabolcs Nagy <szabolcs.nagy@arm.com>
CommitDate: 2018-01-15 11:43:33 +0000

     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.


and

commit d726cb5d3784f7ed96318a2214ea777a86f9faad
Author:     Szabolcs Nagy <szabolcs.nagy@arm.com>
AuthorDate: 2018-01-15 13:06:45 +0000
Commit:     Szabolcs Nagy <szabolcs.nagy@arm.com>
CommitDate: 2018-01-15 13:28:25 +0000

     Fix gdb.texinfo more

     Add incorrectly removed @pindex back.

     gdb/doc/ChangeLog:

             * gdb.texinfo (gdb-add-index man): Add pindex.