bpf, riscv: add BPF JIT for RV64G

      This commit adds a BPF JIT for RV64G.

The JIT is a two-pass JIT, and has a dynamic prolog/epilogue (similar
to the MIPS64 BPF JIT) instead of static ones (e.g. x86_64).

At the moment the RISC-V Linux port does not support
CONFIG_HAVE_KPROBES, which means that CONFIG_BPF_EVENTS is not
supported. Thus, no tests involving BPF_PROG_TYPE_TRACEPOINT,
BPF_PROG_TYPE_PERF_EVENT, BPF_PROG_TYPE_KPROBE and
BPF_PROG_TYPE_RAW_TRACEPOINT passes.

The implementation does not support "far branching" (>4KiB).

Test results:
  # modprobe test_bpf
  test_bpf: Summary: 378 PASSED, 0 FAILED, [366/366 JIT'ed]

  # echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled
  # ./test_verifier
  ...
  Summary: 761 PASSED, 507 SKIPPED, 2 FAILED

Note that "test_verifier" was run with one build with
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y and one without, otherwise
many of the the tests that require unaligned access were skipped.

CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y:
  # echo 1