libbpf: Support sanitization of BTF layout for older kernels

Add a FEAT_BTF_LAYOUT feature check which checks if the
kernel supports BTF layout information.  Also sanitize
BTF if it contains layout data but the kernel does not
support it.  The sanitization requires rewriting raw
BTF data to update the header and eliminate the layout
section (since it lies between the types and strings),
so refactor sanitization to do the raw BTF retrieval
and creation of updated BTF, returning that new BTF
on success.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260326145444.2076244-7-alan.maguire@oracle.com

src/libbpf_internal.h

@@ -396,6 +396,8 @@ enum kern_feature_id {
 	FEAT_LDIMM64_FULL_RANGE_OFF,
 	/* Kernel supports uprobe syscall */
 	FEAT_UPROBE_SYSCALL,
+	/* Kernel supports BTF layout information */
+	FEAT_BTF_LAYOUT,
 	__FEAT_CNT,
 };
 
@@ -422,6 +424,10 @@ int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz);
 int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
 			 const char *str_sec, size_t str_len,
 			 int token_fd);
+int libbpf__load_raw_btf_hdr(const struct btf_header *hdr,
+			     const char *raw_types, const char *str_sec,
+			     const char *layout_sec, int token_fd);
+
 int btf_load_into_kernel(struct btf *btf,
 			 char *log_buf, size_t log_sz, __u32 log_level,
 			 int token_fd);