bfd: use the ELF linker to perform relocations in BPF targets

bfd: use the ELF linker to perform relocations in BPF targets

This patch changes the eBPF linker to provide a relocate_section
function instead of relying on using special functions in relocation
howtos.

Tested in x86_64 host.
No regressions.

bfd/ChangeLog:

2019-08-07  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* elf64-bpf.c (bpf_elf_relocate_section): New function.
	(bpf_elf_insn_disp_reloc): Delete function.
	(elf_backend_relocate_section): Define.