GNU strip fails to set sh_link and sh_info on Solaris SPARC64

The patch adds a missing elf64_sparc_copy_solaris_special_section_fields
function that enables to fill sh_link and sh_info fields in .SUNW_* sections.
Note that elf64_sparc_copy_solaris_special_section_fields is empty since
the default handling is currently sufficient for GNU strip command.

This is a followup patch of the following upstream commits:

commit 5522f910
Author: Nick Clifton <nickc@redhat.com>
Date:   Fri Apr 29 09:24:42 2016 +0100

    Enhance support for copying and stripping Solaris and ARM binaries.

commit 84865015
Author: Nick Clifton <nickc@redhat.com>
Date:   Thu Apr 14 12:04:09 2016 +0100

    Fix copying Solaris binaries with objcopy.

gdb/ChangeLog:

2021-03-01  Libor Bukata <libor.bukata@oracle.com>

        * bfd/elf64-sparc.c: Fix GNU strip on Solaris SPARC64.