bfd: fix the deletion of relocs in sparc64

bfd: fix the deletion of relocs in sparc64

This patch fixes the deletion of relocations in BFD sections in
sparc64 targets.

A specialized `_bfd_set_reloc' function is provided that updates the
internal canon_reloc_count(sec) counter instead of sec->reloc_count.
Additionally, the `write_relocs' callback in elf64-sparc is adapted to
use the canon_reloc_count to traverse `sec->orelocation'.

Tested in sparc64-linux-gnu targets.
Fixes an existing failure in the merge-notes objcopy test.
No regressions.

bfd/ChangeLog:

2017-05-10  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* elf64-sparc.c (elf64_sparc_set_reloc): New function.
	(bfd_elf64_set_reloc): Define.
	(elf64_sparc_write_relocs): Use `canon_reloc_count'.