add missing const on powerpc FE_DFL_ENV

arch/powerpc/bits/fenv.h

@@ -28,4 +28,4 @@
 typedef unsigned fexcept_t;
 typedef double fenv_t;
 
-#define FE_DFL_ENV ((fenv_t *)-1)
+#define FE_DFL_ENV ((const fenv_t *)-1)