bpf: relocation fixes for eBPF ELF backend

The eBPF ELF backend was not properly recording relocation addends
during installation, nor reading and applying them when performing
the final relocation. This lead to various issues with incorrect
relocations.

These issues are fixed with a new howto special function to install
the relocations, and updates to bpf_elf_relocate_section to read and
use the addends as recorded in the input_bfd.

bfd/ChangeLog

2020-08-05  David Faust  <david.faust@oracle.com>

	* elf64-bpf.c (bpf_elf_generic_reloc): New function.
	(bpf_elf_howto_table): Use it here.
	(bpf_elf_relocate_section): Use addends recorded in input_bfd for
	instruction and data relocations.

ld/ChangeLog

2020-08-05  David Faust  <david.faust@oracle.com>

	* testsuite/ld-bpf/call-2.s: New file.
	* testsuite/ld-bpf/call-2.d: Likewise.
	* testsuite/ld-bpf/reloc-data-be.d: Likewise.
	* testsuite/ld-bpf/reloc-data-le.d: Likewise.
	* testsuite/ld-bpf/reloc-data.s: Likewise.
	* testsuite/ld-bpf/reloc-insn-external-be.d: Likewise.
	* testsuite/ld-bpf/reloc-insn-external-le.d: Likewise.
	* testsuite/ld-bpf/reloc-insn-external.s: Likewise.
	* testsuite/ld-bpf/reloc-insn32-be.d: Likewise.
	* testsuite/ld-bpf/reloc-insn32-le.d: Likewise.
	* testsuite/ld-bpf/reloc-insn32.s: Likewise.
	* testsuite/ld-bpf/reloc-insn64-be.d: Likewise.
	* testsuite/ld-bpf/reloc-insn64-le.d: Likewise.
	* testsuite/ld-bpf/reloc-insn64.s: Likewise.