Implement nanl in newlib only

Drop Cygwin-specific nanl in favor of a generic implementation
in newlib.  Requires GCC 3.3 or later.

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

winsup/cygwin/math/nanl.c

@@ -1,5 +0,0 @@
-long double
-nanl (const char *tagp)
-{
-  return __builtin_nanl ("");
-}