gdb: backport an upstream patch to support fission with split debug.

The upstream patch was
commit 6c4474237ab3356b100ebbdd6ff1a8839745023d
Author: Doug Evans <dje@google.com>
Date:   Tue May 3 16:30:58 2016 -0700

    PR symtab/19914 fix handling of dwp + split debug

        PR symtab/19914
        * dwarf2read.c (open_and_init_dwp_file): Look at backlink if
        objfile is separate debug file.

        testsuite/
        * gdb.dwarf2/dwp-sepdebug.c: New file.
        * gdb.dwarf2/dwp-sepdebug.exp: New file.

TEST=build chrome with debug fission and put the .dwp file in the debug
     dir. gdb can read debug information from there.