btf: set .BTF section alignment to 16

This is to avoid misaligned access when memory-mapping ELF sections.

Signed-off-by: Giuliano Procida <gprocida@google.com>

libbtf.c

@@ -796,6 +796,14 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 			goto unlink;
 		}
 
+		snprintf(cmd, sizeof(cmd), "%s --set-section-alignment .BTF=16 %s",
+			 llvm_objcopy, filename);
+		if (system(cmd)) {
+			/* non-fatal, this is a nice-to-have and it's only supported from LLVM 10 */
+			fprintf(stderr, "%s: warning: failed to align .BTF section in '%s': %d!\n",
+				__func__, filename, errno);
+		}
+
 		err = 0;
 	unlink:
 		unlink(tmp_fn);