lib: c: add fallback errno storage for single-threaded programs

lib/libc/include/errno.h

@@ -1,6 +1,8 @@
 #ifndef ERRNO_H_
 #define ERRNO_H_
 
+#define errno           (*__errno_location())
+
 #define SUCCESS         0  /* Success */
 #define EPERM           1  /* Operation not permitted */
 #define ENOENT          2  /* No such file or directory */
@@ -142,4 +144,6 @@
 extern int __set_errno(int err);
 extern int __errno_from_kern_status(unsigned int err);
 
+extern int __attribute__((weak)) * __errno_location(void);
+
 #endif