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 > /proc/sys/kernel/unprivileged_bpf_disabled
# ./test_verifier | grep -c 'NOTE.*unknown align'
0

No CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS:
# echo 1 > /proc/sys/kernel/unprivileged_bpf_disabled
# ./test_verifier | grep -c 'NOTE.*unknown align'
59

The two failing test_verifier tests are:
"ld_abs: vlan + abs, test 1"
"ld_abs: jump around ld_abs"

This is due to that "far branching" involved in those tests.

All tests where done on QEMU (QEMU emulator version 3.1.50
(v3.1.0-688-g8ae951fbc106)).

Signed-off-by: Björn Töpel
Signed-off-by: Daniel Borkmann