fftwQuad: Mark unsupported on aarch64

https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html
https://fftw.org/fftw3_doc/Precision.html

Building fftw with quad precision requires libquadmath via gfortran. On
aarch64, libquadmath is not supported due to GCC's __float128 type not
being supported on that platform.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/libraries/fftw/default.nix

@@ -85,5 +85,7 @@ stdenv.mkDerivation (finalAttrs: {
       }.${precision}
     ];
     platforms = platforms.unix;
+    # quad-precision requires libquadmath from gfortran, but libquadmath is not supported on aarch64
+    badPlatforms = lib.optionals (precision == "quad-precision") platforms.aarch64;
   };
 })