Backport: std.math: Use core.stdc.fenv for RoundingMode values

03b3d7fb302c0d9ebb975d921691b2443059fe82

Author: jpf91

libphobos/src/std/math.d

@@ -5044,34 +5044,25 @@ struct FloatingPointControl
             roundToZero /// ditto
         }
     }
-    else version(ARM)
-    {
-        enum : RoundingMode
-        {
-            roundToNearest = 0x000000,
-            roundDown      = 0x800000,
-            roundUp        = 0x400000,
-            roundToZero    = 0xC00000
-        }
-    }
-    else version(PPC_Any)
+    else version (CRuntime_Microsoft)
     {
+        // Microsoft uses hardware-incompatible custom constants in fenv.h (core.stdc.fenv).
         enum : RoundingMode
         {
-            roundToNearest = 0x00000000,
-            roundDown      = 0x00000003,
-            roundUp        = 0x00000002,
-            roundToZero    = 0x00000001
+            roundToNearest = 0x0000,
+            roundDown      = 0x0400,
+            roundUp        = 0x0800,
+            roundToZero    = 0x0C00
         }
     }
     else
     {
         enum : RoundingMode
         {
-            roundToNearest = 0x0000,
-            roundDown      = 0x0400,
-            roundUp        = 0x0800,
-            roundToZero    = 0x0C00
+            roundToNearest = core.stdc.fenv.FE_TONEAREST,
+            roundDown      = core.stdc.fenv.FE_DOWNWARD,
+            roundUp        = core.stdc.fenv.FE_UPWARD,
+            roundToZero    = core.stdc.fenv.FE_TOWARDZERO,
         }
     }