errno: add the errno variable and base macros

A <bits/errno.h> file will likely by added later on for each target to
add more specific values.

CMakeLists.txt

@@ -73,6 +73,7 @@ set(SOURCES
   src/libc/ctype/isxdigit.c
   src/libc/ctype/tolower.c
   src/libc/ctype/toupper.c
+  src/libc/errno/errno.c
   src/libc/locale/setlocale.c
   src/libc/locale/localeconv.c
   src/libc/stdio/vsnprintf.c

include/errno.h

@@ -0,0 +1,12 @@
+#ifndef __ERRNO_H__
+# define __ERRNO_H__
+
+/* Initialized to 0 at startup. Currently not TLS, maybe in the future if
+   threads are supported. */
+extern int errno;
+
+#define EDOM       1
+#define EILSEQ     2
+#define ERANGE     3
+
+#endif /*__ERRNO_H__*/

src/libc/errno/errno.c

@@ -0,0 +1,4 @@
+#include <errno.h>
+
+/* The initial value is 0 at program startup */
+int errno = 0;