Rename __sentinel to __null_sentinel

GCC 5 uses a conflicting __sentinel definition in include/c++/bits/stl_algo.h

Reported by:	matteo

newlib/libc/include/sys/cdefs.h

@@ -464,7 +464,7 @@
 #endif
 
 #if __GNUC_PREREQ__(4, 0)
-#define	__sentinel	__attribute__((__sentinel__))
+#define	__null_sentinel	__attribute__((__sentinel__))
 #define	__exported	__attribute__((__visibility__("default")))
 /* Only default visibility is supported on PE/COFF targets. */
 #ifndef __CYGWIN__
@@ -473,7 +473,7 @@
 #define	__hidden
 #endif
 #else
-#define	__sentinel
+#define	__null_sentinel
 #define	__exported
 #define	__hidden
 #endif