cygwin/math: make isinf functions signed

glibc returns -1 for negative infinity:

http://man7.org/linux/man-pages/man3/isinfl.3.html
https://sourceware.org/bugzilla/show_bug.cgi?id=15367

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/math/isinf.c

@@ -1,18 +1,18 @@
 int
 isinf (double x)
 {
-  return __builtin_isinf (x);
+  return __builtin_isinf_sign (x);
 }
 
 int
 isinff (float x)
 {
-  return __builtin_isinf (x);
+  return __builtin_isinf_sign (x);
 }
 
 int
 isinfl (long double x)
 {
-  return __builtin_isinf (x);
+  return __builtin_isinf_sign (x);
 }