ntdll: Copy floor() implementation from msvcrt.

dlls/ntdll/math.c

@@ -191,10 +191,31 @@ double CDECL fabs( double d )
 
 /*********************************************************************
  *                  floor   (NTDLL.@)
+ *
+ * Based on musl: src/math/floorf.c
  */
-double CDECL floor( double d )
+double CDECL floor( double x )
 {
-    return unix_funcs->floor( d );
+    union {double f; UINT64 i;} u = {x};
+    int e = (int)(u.i >> 52 & 0x7ff) - 0x3ff;
+    UINT64 m;
+
+    if (e >= 52)
+        return x;
+    if (e >= 0) {
+        m = 0x000fffffffffffffULL >> e;
+        if ((u.i & m) == 0)
+            return x;
+        if (u.i >> 63)
+            u.i += m;
+        u.i &= ~m;
+    } else {
+        if (u.i >> 63 == 0)
+            return 0;
+        else if (u.i << 1)
+            return -1;
+    }
+    return u.f;
 }
 
 /*********************************************************************