openat2: create a weak syscall wrapper for it

openat2 allows closing more TOCTOU holes, but we can only use it when
the kernel supports it.

This should disappear seamlessly when libc implements the function.

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

include/crucible/openat2.h

@@ -0,0 +1,17 @@
+#ifndef CRUCIBLE_OPENAT2_H
+#define CRUCIBLE_OPENAT2_H
+
+#include <linux/openat2.h>
+
+#include <fcntl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+extern "C" {
+
+/// Weak symbol to support libc with no syscall wrapper
+int openat2(int dirfd, const char *pathname, struct open_how *how, size_t size) throw();
+
+};
+
+#endif // CRUCIBLE_OPENAT2_H