folly: fix build on aarch64-linux

pkgs/development/libraries/folly/default.nix

@@ -1,4 +1,5 @@
-{ lib, stdenv
+{ lib
+, stdenv
 , fetchFromGitHub
 , boost
 , cmake
@@ -57,7 +58,13 @@ stdenv.mkDerivation rec {
   propagatedBuildInputs = lib.optional stdenv.isLinux jemalloc;
 
   NIX_CFLAGS_COMPILE = [ "-DFOLLY_MOBILE=${if follyMobile then "1" else "0"}" "-fpermissive" ];
-  cmakeFlags = [ "-DBUILD_SHARED_LIBS=ON" ];
+  cmakeFlags = [
+    "-DBUILD_SHARED_LIBS=ON"
+
+    # temporary hack until folly builds work on aarch64,
+    # see https://github.com/facebook/folly/issues/1880
+    "-DCMAKE_LIBRARY_ARCHITECTURE=${if stdenv.isx86_64 then "x86_64" else "dummy"}"
+  ];
 
   postFixup = ''
     substituteInPlace "$out"/lib/pkgconfig/libfolly.pc \