fixes #6606 (#6610)

tests/float/tfloatnan.nim

@@ -0,0 +1,22 @@
+discard """
+  file: "tfloatnan.nim"
+  output: '''Nim: nan
+Nim: nan (float)
+C: nan (float)
+Nim: nan (double)
+C: nan (double)
+'''
+"""
+
+proc printf(formatstr: cstring): int {.importc: "printf", varargs, header: "<stdio.h>".}
+
+let f = NaN
+echo "Nim: ", f
+
+let f32: float32 = NaN
+echo "Nim: ", f32, " (float)"
+discard printf("C: %f (float)\n", f32)
+
+let f64: float64 = NaN
+echo "Nim: ", f64, " (double)"
+discard printf("C: %lf (double)\n", f64)