[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;                          \
  -                     .long ((%l[l_break] - 1b - 8) / 8) & 0xffff;      \
  -                     .short 0"                         \
  +        asm volatile goto("may_goto %l[l_break]"       \
                        :::: l_break);                    \
          goto l_continue;                                \
          l_break: break;
```
and ran the selftest with the latest llvm with this patch. All tests are
passed.

[1]
https://lore.kernel.org/bpf/20240306031929.42666-1-alexei.starovoitov@gmail.com/
[2]
https://lore.kernel.org/bpf/20240306031929.42666-2-alexei.starovoitov@gmail.com/
[3]
https://lore.kernel.org/bpf/20240306031929.42666-4-alexei.starovoitov@gmail.com/

0 files changed, 0 insertions, 0 deletions