Make math.isNaN,copySign,etc available on objc (#24025)

fixes #23922

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Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

lib/pure/math.nim

@@ -64,7 +64,7 @@ when defined(nimPreviewSlimSystem):
   import std/assertions
 
 
-when defined(c) or defined(cpp):
+when not defined(js) and not defined(nimscript): # C
   proc c_isnan(x: float): bool {.importc: "isnan", header: "<math.h>".}
     # a generic like `x: SomeFloat` might work too if this is implemented via a C macro.