Fix __getreent function for Cygwin

So far the lib function __getreent always returned _impure_ptr.  On Cygwin
this is only correct after _impure_ptr got initialized.  The inline
function in include/cygwin/config.h always returns the right _reent ptr,
though.

After introducing per-thread locales, the __getreent function is called
prior to initialization of _impure_ptr (from dll_crt0_0) to access the
locale pointer, which leads to a crash.

Fix the __getreent lib function for Cygwin to return the correct _reent
pointer all the time.  Rename inline function to __inline_getreent
and introduce a macro __getreent calling the inline function.  Change
the lib function __getreent to call __inline_getreent on Cygwin.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

newlib/libc/reent/getreent.c

@@ -1,5 +1,4 @@
 /* default reentrant pointer when multithread enabled */
-
 #include <_ansi.h>
 #include <reent.h>
 
@@ -10,5 +9,11 @@
 struct _reent *
 _DEFUN_VOID(__getreent)
 {
+#ifdef __CYGWIN__
+  /* Utilize Cygwin's inline definition from include/cygwin/config.h
+     (note the extra underscore) */
+  return __inline_getreent ();
+#else
   return _impure_ptr;
+#endif
 }