openat2: supply the missing definitions for building with old headers and new kernel

Apparently Ubuntu 20 has upgraded to kernel 5.15, but still builds things
with 5.4 headers.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/openat2.cc

@@ -1,5 +1,27 @@
 #include "crucible/openat2.h"
 
+#include <sys/syscall.h>
+
+// Compatibility for building on old libc for new kernel
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 6, 0)
+
+// Every arch that defines this uses 437, except Alpha, where 437 is
+// mq_getsetattr.
+
+#ifndef SYS_openat2
+#ifdef __alpha__
+#define SYS_openat2 547
+#else
+#define SYS_openat2 437
+#endif
+#endif
+
+#endif // Linux version >= v5.6
+
+#include <fcntl.h>
+#include <unistd.h>
+
 extern "C" {
 
 int
@@ -7,7 +29,12 @@ __attribute__((weak))
 openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size)
 throw()
 {
+#ifdef SYS_openat2
 	return syscall(SYS_openat2, dirfd, pathname, how, size);
+#else
+	errno = ENOSYS;
+	return -1;
+#endif
 }
 
 };