bfd: new BFD target entry point _bfd_set_reloc.

This patch adds a new entry point to the BFD_JUMP_TABLE_RELOCS.  The
previous common implementation `bfd_set_reloc', in bfd/bfd.c, has been
moved to bfd/reloc.c with the name `_bfd_generic_set_reloc', and all
BFD targets has been adapted to use it.

This patch doesn't introduce any change on functionality, but prepares
the ground for further work.

bfd/ChangeLog:

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

    	* targets.c (BFD_JUMP_TABLE_RELOCS): Add NAME##_set_reloc.
    	(struct bfd_target): New field _bfd_set_reloc.
    	* bfd.c (bfd_set_reloc): Call backend _set_bfd.
    	* reloc.c (_bfd_generic_set_reloc): New function.
    	* coffcode.h (coff_set_reloc): Define to _bfd_generic_set_reloc.
    	* nlm-target.h (nlm_set_reloc): Likewise.
    	* coff-rs6000.c (_bfd_xcoff_set_reloc): Likewise.
    	* aout-tic30.c (MY_set_reloc): Likewise.
    	* aout-target.h (MY_set_reloc): Likewise.
    	* elfxx-target.h (bfd_elfNN_set_reloc): Likewise.
    	* coff-alpha.c (_bfd_ecoff_set_reloc): Likewise.
    	* mach-o-target.c (bfd_mach_o_set_reloc): Likewise.
    	* vms-alpha.c (alpha_vms_set_reloc): Likewise.
    	* aout-adobe.c (aout_32_set_reloc): Likewise.
    	* bout.c (b_out_set_reloc): Likewise.
    	* coff-mips.c (_bfd_ecoff_set_reloc): Likewise.
    	* i386os9k.c (aout_32_set_reloc): Likewise.
    	* ieee.c (ieee_set_reloc): Likewise.
    	* oasys.c (oasys_set_reloc): Likewise.
    	* som.c (som_set_reloc): Likewise.
    	* versados.c (versados_set_reloc): Likewise.
    	* coff64-rs6000.c (rs6000_xcoff64_vec): Add
    	_bfd_generic_set_reloc.
    	(rs6000_xcoff64_aix_vec): LIkewise.
    	* libbfd.c (_bfd_norelocs_set_reloc): New function.
    	* libbfd-in.h: Prototype for _bfd_norelocs_set_reloc.
    	* i386msdos.c (msdos_set_reloc): Define to
    	_bfd_norelocs_set_reloc.
    	* elfcode.h (elf_set_reloc): Define.
    	* bfd-in2.h: Regenerated.

bfd/coffcode.h

@@ -5439,6 +5439,10 @@ coff_canonicalize_reloc (bfd * abfd,
   return section->reloc_count;
 }
 
+#ifndef coff_set_reloc
+#define coff_set_reloc _bfd_generic_set_reloc
+#endif
+
 #ifndef coff_reloc16_estimate
 #define coff_reloc16_estimate dummy_reloc16_estimate