lib: c: re-organise into separate static modules, plus a single shared library

lib/libc/io/unistd/mmap.c

@@ -0,0 +1,6 @@
+#include <sys/mman.h>
+
+void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset)
+{
+	return NULL;
+}