newlib: Fix header issues with C++ guards and implicit inclusion

* Added C++ sentinels if missed
* Used #pragma once, removed macro's with leading underscores
* Updated copyright checker to allow "BSD-2-Clause-FreeBSD AND
Apache-2.0" for newlib files
* Fixed minor compilation issues/implicit inclusions

components/newlib/platform_include/sys/reent.h

@@ -8,8 +8,16 @@
 
 #include_next<sys/reent.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* This function is not part of the newlib API, it is defined in libc/stdio/local.h
  * There is no nice way to get __cleanup member populated while avoiding __sinit,
  * so extern declaration is used here.
  */
 extern void _cleanup_r(struct _reent* r);
+
+#ifdef __cplusplus
+}
+#endif