bpf: fix relocation addend incorrect symbol value

bpf: fix relocation addend incorrect symbol value

Relocations installed by the BPF ELF backend were sometimes incorrectly
adding the symbol value to the relocation entry addend, when the correct
relocation value was already stored in the addend. This could lead to a
relocation effectively adding the symbol value twice.

Fix that by making bpf_elf_generic_reloc () more similar to the flow of
bfd_install_relocation in the case where howto->install_addend is set,
which is how it ought to behave.

bfd/
	* bpf-reloc.def (R_BPF_64_ABS32, R_BPF_64_ABS64)
	(R_BPF_64_NODYLD32): Set partial_inplace to true.
	* elf64-bpf.c (bpf_elf_generic_reloc): Do not include the value
	of the symbol when installing relocation. Copy some additional
	logic from bfd_elf_generic_reloc.

gas/
	* testsuite/gas/bpf/bpf.exp: Run new test.
	* testsuite/gas/bpf/elf-relo-1.d: New.
	* testsuite/gas/bpf/elf-relo-1.s: New.