selftests/bpf: __imm_insn & __imm_const macro for bpf_misc.h

selftests/bpf: __imm_insn & __imm_const macro for bpf_misc.h

Add two convenience macro for BPF test cases,
allowing the following usage:

  #include <linux/filter.h>

  ...
  asm volatile (
  ...
  ".8byte %[raw_insn];"
  ...
  "r1 += %[st_foo_offset];"
  ...
  :
  : __imm_insn(raw_insn, BPF_RAW_INSN(...)),
    __imm_const(st_foo_offset, offsetof(struct st, foo))
  : __clobber_all);

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230325025524.144043-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>