libbpf: Add support for extended BPF syscall

To support the extended BPF syscall introduced in the previous commit,
introduce the following internal APIs:

* 'sys_bpf_ext()'
* 'sys_bpf_ext_fd()'
  They wrap the raw 'syscall()' interface to support passing extended
  attributes.
* 'probe_sys_bpf_ext()'
  Check whether current kernel supports the BPF syscall common attributes.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260512153157.28382-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/libbpf_internal.h

@@ -398,6 +398,8 @@ enum kern_feature_id {
 	FEAT_UPROBE_SYSCALL,
 	/* Kernel supports BTF layout information */
 	FEAT_BTF_LAYOUT,
+	/* Kernel supports BPF syscall common attributes */
+	FEAT_BPF_SYSCALL_COMMON_ATTRS,
 	__FEAT_CNT,
 };
 
@@ -768,4 +770,5 @@ int probe_fd(int fd);
 #define SHA256_DWORD_SIZE SHA256_DIGEST_LENGTH / sizeof(__u64)
 
 void libbpf_sha256(const void *data, size_t len, __u8 out[SHA256_DIGEST_LENGTH]);
+int probe_sys_bpf_ext(void);
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */