[BPF] Add support for may_goto insn (#85358)

Alexei added may_goto insn in [1]. The asm syntax for may_goto looks
like
  may_goto <label>

The instruction represents a conditional branch but the condition is
implicit. Later in bpf kernel verifier, the 'may_goto <label>' insn will
be rewritten with an explicit condition. The encoding of 'may_goto' insn
is enforced in [2] and is also implemented in this patch.

In [3], 'may_goto' insn is encoded with raw bytes. I made the following
change
```
  --- a/tools/testing/selftests/bpf/bpf_experimental.h
  +++ b/tools/testing/selftests/bpf/bpf_experimental.h
  @@ -328,10 +328,7 @@ l_true:                                                                                            \

   #define cond_break                                     \
          ({ __label__ l_break, l_continue;               \
  -        asm volatile goto("1:.byte 0xe5;                       \
  -                     .byte 0;                          \
  -                  ...