G M: Restore the ability for libcxx to compile again on mingw 64.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@190837 91177308-0d34-0410-b5e6-96231b3b80d8

include/support/win32/math_win32.h

@@ -16,7 +16,9 @@
 #else
 
 #include <math.h>
+#include <float.h> // _FPCLASS_PN etc.
 
+// Necessary?
 typedef float float_t;
 typedef double double_t;