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>

lib/Makefile

@@ -14,6 +14,7 @@ CRUCIBLE_OBJS = \
 	fs.o \
 	multilock.o \
 	ntoa.o \
+	openat2.o \
 	path.o \
 	process.o \
 	string.o \