selftests/bpf: Using llvm may_goto inline asm for cond_break macro

Currently, cond_break macro uses bytes to encode the may_goto insn.
Patch [1] in llvm implemented may_goto insn in BPF backend.
Replace byte-level encoding with llvm inline asm for better usability.
Using llvm may_goto insn is controlled by macro __BPF_FEATURE_MAY_GOTO.

  [1] https://github.com/llvm/llvm-project/commit/0e0bfacff71859d1f9212205f8f873d47029d3fbSigned-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20240402025446.3215182-1-yonghong.song@linux.dev

tools/testing/selftests/bpf/bpf_experimental.h

@@ -326,6 +326,16 @@ l_true:												\
        })
 #endif
 
+#ifdef __BPF_FEATURE_MAY_GOTO
+#define cond_break					\
+	({ __label__ l_break, l_continue;		\
+	 asm volatile goto("may_goto %l[l_break]"	\
+		      :::: l_break);			\
+	goto l_continue;				\
+	l_break: break;					\
+	l_continue:;					\
+	})
+#else
 #define cond_break					\
 	({ __label__ l_break, l_continue;		\
 	 asm volatile goto("1:.byte 0xe5;			\
@@ -337,6 +347,7 @@ l_true:												\
 	l_break: break;					\
 	l_continue:;					\
 	})
+#endif
 
 #ifndef bpf_nop_mov
 #define bpf_nop_mov(var) \