Missing support for RISC-V architecture (64-bit)

[xmzzz]

Hi, I followed the steps in README on the riscv64 architecture and encountered an error. The reason is that the vmlinux.h file on riscv64 platform is missing. I will create a PR later to solve this problem.

The system information and error message are as follows:

ubuntu@ubuntu:~/work/libbpf-bootstrap/examples/c$ uname -a
Linux ubuntu 5.19.0-1012-generic #13~22.04.1-Ubuntu SMP Thu Jan 12 15:34:31 UTC 2023 riscv64 riscv64 riscv64 GNU/Linux
ubuntu@ubuntu:~/work/libbpf-bootstrap/examples/c$ make minimal
  MKDIR    .output
  CC       .output/minimal.bpf.o
  MKDIR    bpftool
  BPFTOOL  bpftool/bootstrap/bpftool
...                        libbfd: [ on  ]
...        disassembler-four-args: [ on  ]
...                          zlib: [ on  ]
...                        libcap: [ on  ]
...               clang-bpf-co-re: [ on  ]
  MKDIR    /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/include/bpf
  INSTALL  /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/include/bpf/hashmap.h
  INSTALL  /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/include/bpf/relo_core.h
  INSTALL  /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/include/bpf/libbpf_internal.h
  MKDIR    /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/
  MKDIR    /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/
  MKDIR    /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/bpf.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/btf.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/libbpf.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/libbpf_errno.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/netlink.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/nlattr.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/str_error.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/libbpf_probes.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/bpf_prog_linfo.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/btf_dump.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/hashmap.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/ringbuf.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/strset.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/linker.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/gen_loader.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/relo_core.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/staticobjs/usdt.o
  AR       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/libbpf/libbpf.a
  INSTALL  bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h bpf_helpers.h bpf_helper_defs.h bpf_tracing.h bpf_endian.h bpf_core_read.h skel_internal.h libbpf_version.h usdt.bpf.h
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/main.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/common.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/json_writer.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/gen.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/btf.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/xlated_dumper.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/btf_dumper.o
  CC       /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/disasm.o
  LINK     /home/ubuntu/work/libbpf-bootstrap/examples/c/.output/bpftool/bootstrap/bpftool
  GEN-SKEL .output/minimal.skel.h
libbpf: elf: minimal_bpf is not a valid eBPF object file
Error: failed to open BPF object file: BPF object format invalid
make: *** [Makefile:112: .output/minimal.skel.h] Error 95
make: *** Deleting file '.output/minimal.skel.h'