[libc] Update errno and strto* files for compatibility

Most of the strto* functions are only stubbed out, but they're there for
things that link to them but don't call them.

src/libraries/libc/include/arch/amd64/errno.h

@@ -2,4 +2,13 @@
 /// \file arch/amd64/errno.h
 /// errno implementation for amd64
 
-extern int errno;
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int * __errno_location();
+#define errno (*__errno_location())
+
+#ifdef __cplusplus
+} // extern C
+#endif