newlib: remove __infinity{f,ld} constants

previous commit 4c90db7bc8 introduced
a compile time error because libm/common/s_infconst.c used the remove
__fmath, __dmath, and __ldmath union types.

Since this is very old, and unused for a very long time, just drop the
file and thus the __infinity constants entirely.

Exception: Cygwin exports __infinity from the beginning.  There's a very,
VERY low probability that any existing executable or lib still uses this
constant, but we just keep it in for backward compat, nevertheless.

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

winsup/cygwin/math/isinf.c

@@ -16,3 +16,8 @@ isinfl (long double x)
   return __builtin_isinf_sign (x);
 }
 
+/* Infinity as a constant value.  Should never actually be used any longer,
+   but has been exported by Cygwin for ages so we keep it here for (probably
+   unnecessary) backward compat. */
+#include <float.h>
+const double __infinity[1] = { DBL_MAX+DBL_MAX };