Add support for signed DIV and MOD instructions

[dthaler]

BPF_SDIV
BPF_SMOD

Documentation references:

• ietf-wg-bpf/ebpf-docs#42
• https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/Documentation/bpf/standardization/instruction-set.rst?id=245d4c40c09bd8d5a71640950eeb074880925b9a