Use weak reference for _REENT_SMALL

Avoid a strong reference to __sfp[] for _impure_data.  The __sfp[] is linked in
if __sinit() is used for example.

newlib/libc/reent/impure.c

@@ -6,10 +6,14 @@
    important to reduce image size for targets with very small amounts
    of memory.  */
 #ifdef _REENT_SMALL
+#ifdef _REENT_GLOBAL_STDIO_STREAMS
+extern __FILE __sf[3] _ATTRIBUTE ((weak));
+#else
 extern const struct __sFILE_fake __sf_fake_stdin _ATTRIBUTE ((weak));
 extern const struct __sFILE_fake __sf_fake_stdout _ATTRIBUTE ((weak));
 extern const struct __sFILE_fake __sf_fake_stderr _ATTRIBUTE ((weak));
 #endif
+#endif
 
 struct _reent __ATTRIBUTE_IMPURE_DATA__ _impure_data = _REENT_INIT (_impure_data);
 #ifdef __CYGWIN__